Naming such syntax as convertible propositions and saying that the selection is a matter of taste and convention, does not help us much in understanding the meaning of the passage. I am giving a different answer than what you asked for, but it might be of interest to some people.
I discussed this passage in the article "The eye is the lamp of the body".
If we talk about semantics and the meaning of the metaphors of eye, body and lamp, we need to recognize that Greek puts the most prominent word first (English usually puts it last). The focus is on spiritual light, represented by the illustration of a lamp. Another aspect is whether a person is open to allow that spiritual light to enter his mind (Hebrew would say his heart) or whether he blocks it. It is easier to see this if we look at the whole passage:
Ὁ λύχνος τοῦ σώματός ἐστιν ὁ ὀφθαλμός. ἐὰν οὖν ᾖ ὁ ὀφθαλμός σου ἁπλοῦς, ὅλον τὸ σῶμά σου φωτεινὸν ἔσται· ἐὰν δὲ ὁ ὀφθαλμός σου πονηρὸς ᾖ, ὅλον τὸ σῶμά σου σκοτεινὸν ἔσται. εἰ οὖν τὸ φῶς τὸ ἐν σοὶ σκότος ἐστίν, τὸ σκότος πόσον.
The lamp/light of the body is the eye. So, if your eye might be single, your whole body will be shining (have light). But if your eye might be bad, your whole body will be dark. So if the light that is in you is darkness, what great darkness!
Hebrew often uses a chiasm where the beginning (Ὁ λύχνος τοῦ σώματός) and end (τὸ φῶς τὸ ἐν σοὶ) go together. In the center we have the contrast between a single and a bad eye. To show this, I prefer to translate the first word as light rather than lamp as even KJV does. The eye here is like a window into the inner person. It stands for an attitude of mind. If you are singleminded (ἁπλοῦς) in the positive sense of being focused on Jesus and the light he brings, then your will have spiritual light inside you. But if your "eye" is bad, that is, your mind is unfocused or focused on wrong things, then you are not allowing the light of Jesus to enter your mind.
It is very difficult to get that meaning across in a literal translation. My own idiomatic translation (The Bible in Everyday Danish - Bibelen på Hverdagsdansk, first published in 2007) is here:
Øjet er som en lampe, der bringer lys til dit indre. Hvis du har et åbent og oprigtigt sind, vil dit indre fyldes af mit lys. Men hvis du har et lukket og ondt sind, vil dit indre fyldes af mørke. Pas på ikke at lukke af for mit lys, for så bliver dit liv ét stort mørke.
Let me give it in English:
The eye is like a lamp that brings light to your inner being. If you have an open and sincere mind, your inner being will be filled by my light. But if you have a closed and evil mind, your inner being will be filled with darkness. Beware not to close out my light, for then your life will be one big darkness.
There are a few very free translations in English that also try to communicate the meaning as best they can. I am not too fond of any of them, but let me quote them anyway:
“The eyes of your spirit allow revelation-light to enter into your being. If your heart is unclouded, the light floods in! But if your eyes are focused on money, the light cannot penetrate and darkness takes its place. How profound will be the darkness within you if the light of truth cannot enter! (Passion translation.)
“Your eyes are windows into your body. If you open your eyes wide in wonder and belief, your body fills up with light. If you live squinty-eyed in greed and distrust, your body is a musty cellar. If you pull the blinds on your windows, what a dark life you will have! (The Message)