I wouldn't say its opposites or focus so much on the wording, but rather the thematic focus of each of the prophets.
A big emphasis of Isaiah is the salvation of Israel. Isaiah's name means "The LORD saves", and the word for salvation is used 30 times in the book of Isaiah. Isaiah prophesies more about the coming Messiah than any other prophet in the Bible, and the entire latter portion of the book is titled "The Book of Comforts". Although Isaiah, like the other prophets, does also warn of the coming judgment of God, he also speaks greatly of God as the Savior of the world.
The emphasis of Joel, however, is on the judgment of God, specifically the Day of the LORD. Joel uses the phrase "Day of the LORD" five times throughout just three chapters, which is the most than any other book in the Bible. Joel, along with the book of Zephaniah, is one of the two minor prophets that deal exclusively with the Day of the LORD and coming judgment.
Let's also address the context of the passages:
In Isaiah, the passage you described, it speaks of the peace that is to come after God comes and judges the world. In Joel, it talks of God's judgment on that day, where he "arouses the nations". Basically, God is going to rouse the wicked nations and they will take arms against him. God quickly defeats (duh) and there will be no more war and conflict after this. God reigns supreme.