As has also been rehashed elsewhere, the 320M still sucks for hardcore gaming. It's a case that it sucks slightly less.
Both of them are just fine for light gaming.
If you want thin-and-light-and-sucks-at-games, buy the Air. If you want full-featured-but-still-sucks-at-games, buy the 13" Pro. If you want fully-featured, buy the 15"+ Pro.
The Intel graphics will play WoW about the same as the 320M, it will play other "medium" games about the same (yeah, you'll run at 23 FPS instead of 28 FPS, but it's still FPS-in-the-20s.) Both suck at higher end games, and both will play light games perfectly well.
But if you do things like movie transcoding, or compiling, or very large spreadsheet recalculations, the Pro will kick the Air's ass.