I believe they are the same person, but that the two events took place at different times in Jesus' ministry.
There are several reasons to believe this is true.
First, there are very good reasons to believe the event described in Luke 7 took place in Bethany of Judea, and not in Galilee as is often assumed. Consider the fact that before Jesus raised the widow's son in Nain, he had been in Capernaum, just one day earlier. Capernaum was located 20 or 30 miles north of Nain, which means he traveled a considerable distance south in just one day. Now consider that immediately AFTER he raised the widow's son in Nain, certain disciples of John the Baptist, who were eye-witnesses to the miracles that had taken place, left the region to return to the place where John was being kept in prison. (The fact that John was in prison at this time is evident because of Matt 4:12.) Now it says that after they reported to him those things that they had witnessed he sent two of his disciples to inquire of Jesus as to whether he was the Messiah, or whether they should expect another. The reason all of this is significant is because of WHERE John was being kept in prison. Josephus tells us that John was imprisoned and eventually beheaded at a hilltop fortress of Herod, located about 9 miles east of the Red Sea on the other side of the Jordan River. That was a long way from Southern Galilee, which is where Nain was located. Since Jesus was already traveling south when the disciples of John left initially, it is reasonable to think that Jesus could have traveled through or around Samaria and made his way to Bethany of Judea well before John's disciples arrived at Herod's fortress, and certainly before the two disciples John sent to inquire of Jesus made their way to Him. It was after they departed that Jesus spoke glowingly of John the Baptist and he was invited by a Pharisee to have dinner at his home. The pharisee's name is Simon.
So, having established Bethany as a possible location for the event recorded in Luke 7, we have to determine if Bethany is the most likely location, and whether or not it makes sense to link this event in Luke 7 with the later event described in Mk, Mt, and Jn. If that is the case, we can safely conclude that the two Simon's are the same.
We have a very, very good reason to believe that these two events are linked.
Aware of this, Jesus said to them, “Why are you bothering this woman? She has done a beautiful thing to me. The poor you will always have with you, but you will not always have me. When she poured this perfume on my body, she did it to prepare me for burial. Truly I tell you, wherever this gospel is preached throughout the world, what she has done will also be told, in memory of her.”
If Jesus Himself said that this woman's actions were of such importance that the testimony of what she did would be proclaimed wherever the gospel is preached throughout the whole world, it makes ZERO sense that Luke would leave it out of his gospel. So here is what I have concluded.
Not only are the two Simon's the same person, but the sinful woman of Luke 7 is none other than Mary of Bethany. The significance of the story is not in a single act of devotion. The act itself is significant because it tells the story of one person's journey from sinner to saint... it points to the transforming power of God's grace and the reality of Christ's unconditional love for everyone who believes in Him.
So let me fill in the blanks. In Luke 7, Simon was offended by Jesus and doubted that He was a prophet because he allowed a sinful woman to touch him. How did he know she was a sinful woman? Because he knew her. Since Martha was recorded as the one serving in Luke 10 and John 13, we know that she was the senior lady in the house, which means that she was either the wife of Simon, or the daughter of Simon. Mary was Martha's sister. Since Martha yells at Mary in Luke 10 for not helping her serve, and we know that they are sisters, it appears they may have shared in this responsibility, which lends support to the notion that Mary and Martha were the daughters of Simon, although it doesn't rule out the possibility that Martha and Simon were married. In any case, Mary, Martha, and Simon were related. Although the Bible doesn't tell us her sin, it doesn't matter because Simon the pharisee, being very concerned with his family's reputation, most likely shamed her publicly and put her out of the house. A woman shamed in public might feel compelled to leave the area, so perhaps she fled to somewhere in Galilee. (My personal hypothesis is that she fled to Magdala, a coastal town located on the Sea of Galilee... Just south of Capernaum on the way to Nain.) What if, while she was there, burdened by all her guilt, weighed down by all her shame, Jesus came passing through her little town. And what if demons began to cry out as he passed by, and what if he cast out 7 seven unclean spirits from her? In other words, what if this woman was also Mary Magdalene, out of whom came 7 unclean spirits? I think it's more likely than not. The passage after the event in Luke 7 is the first time Mary Magdalene is mentioned. Something I find to be significant.
So this sinful woman follows Jesus from Magdala to Nain as he makes his way south, just waiting for the crowds to disperse so she can have a chance to get close to him... so that she could express her gratitude for the kindness shown to her. She follows him all the way back to her home town, to the home of her relatives, and despite the unbelievable shame she must have felt, she rushes in there with an alabaster jar full of ointment, and she hurries over to Jesus, and before she can be kicked out, she breaks it open and annoints him with the oil... and she is so overcome with the shame she feels for her sin, and the love and admiration she feels for Jesus that she begins to weep uncontrollably, soaking our Lord's feet with her tears and wiping them with her hair. She even kisses his feet. He turns to her and says, "Daughter, your sins are forgiven." Imediately, her shame is lifted off of her, shame that had felt like a million pounds on her shoulders. She is happy again, and she commits herself to support the Lord in His ministry out of her own substance. She is a changed woman, and as the Lord ministers in the cities and towns nearby before returning to Galilee, she reconnects with her family. Now at some point, Simon the pharisee contracts leprosy and is forced to move to a leper colony, and Martha and Lazarus, who are both followers of Jesus, accept Mary back into the house. Now it's possible that Simon the leper remained in the leper colony, or it's possible that Jesus healed him and he returned a changed man, but either way, Mary had her family back and her life was radically changed because of Jesus... and then Lazarus dies. She can't stop wondering why the Lord wasn't there. If he were there, Lazarus wouldn't have died. Then she gets the news that Jesus had come. She runs out to meet him. "Lord," she says, "You could have stopped this from happening." Her hurt and confusion touched the heart of God. Jesus wept. He asks them to take Him to the tomb. He tells them to roll the stone away. They protest. "He's been dead for four days. There will be a stench!" He turns and looks at Mary, and he says to the crowd, "I Am the Ressurrection and the Life! Lazarus, come out!" Then her brother comes out in his grave clothes and begins tearing them off and she cries in joy and amazement at the miracle that had just taken place. Her brother who was dead had been restored to her, and for the second time, Jesus had completely changed her life. So for a second time, she planned to do something special to express her gratitude for what He had done. Not long after this, they held a feast in Jesus' honor. It was at her home, the home of Mary and Martha and Lazarus... which was also the home of Simon the leper. Once again she took an alabaster jar full of costly ointment and she broke it and annointed the head and the feet of Jesus. For the second time, He had changed her life and for the second time she showered him with love and devotion. Her heart was full. She had to do something to show him how much she loved him. She had been forgiven much, and so she loved much. Judas protested, "That was expensive! You could have sold it and given the money to the poor!" But Jesus said, "she has done a good thing to me. Mary gets it. Wherever my gospel is preached throughout the whole world, the story of what she has done will be told in memory of her."
But the story doesn't end there. Remember that when Jesus corrected Judas after she annointed Him the second time, Jesus said, “Leave her alone, so that she may keep it for the day of my burial." - Jn 12:7
So after Jesus was crucified, remembering vividly what He had said just days earlier, she again took the alabaster jar full of ointment, what was left after she had annointed Him 5 days before,, and she went to the tomb very early while it was still dark, and she brought with her the other Mary, the mother of Jesus, Solome, Joanna, and Mary the mother of James. She was going to annoint Him again... for the third time... but this time with a VERY heavy heart. They found the stone rolled away. Being perplexed at what could have happened to the body a rush of emotion overwhelms her, and she hurries off without the others to tell Simon Peter and the others what had happened. She tells them, "Someone has taken his body!" Meanwhile, the women who stayed at the tomb we're visited by two angels. One of them spoke and told them Jesus had risen and they fled out of fear. (Jesus would eventually appear to them and they would go to tell the apostles but not before he appeared to someone else first.) John gets to the tomb first after the women had departed but he doesn't go in. Then Peter arrives and hurries into
the tomb. He sees that Jesus isn't there but his headcloth is and it was folded neatly along with His burial clothes. John also goes in after him and sees the same. Confused and unsure what to make of everything they leave, but Mary doesn't. She goes into the empty tomb and weeps. The angels return and one of them says, why are you weeping? Not Not realizing she was speaking to an angel, because her tears were uncontrollable, she says, "Because somebody has taken the Body of my Lord and I don't know where to find Him!" The tears continued to soak the ground... an endless flow of pain was pouring out of her. It was bad enough that He was dead, but now she couldn't even annoint His body and honor Him one last time? She hears a voice behind her... "Woman, why are you weeping?" Looking through her tears in the dim light of dawn, she sees the figure of a man and assumes it's the gardener. "Where have they taken my Lord? Please tell me. Where have they taken His body? Take me to Him, and I will bring Him back. Please, just tell me where He is!". He smiles and says, "Mary." Realizing now that she is looking at the Ressurrected Lord, she runs to embrace Him. He turned her tears of saddness into tears of joy. "Rabboni! You've come back to me! You're alive! You're alive! Just like the widow's son... and just like my brother Lazarus! You came back. You ARE the Messiah!" Jesus said, "Yes, but I'm not here to stay. Go and tell my disciples that I have risen, and that I'm going to return to My Father and YOUR Father... to My God and to YOUR God." A new joy filled her heart, and a new hope came alive. Jesus had changed her life twice already, but this new thing that He had done, this would change EVERYTHING, for EVERYONE... FOREVER.