Well, since I've never even heard of a game called Super Mario Adventure 2, I'd go with Metroid Prime. From what I've heard and read, the game is pretty much flawless...
OH...you're talking about GBA...I thought maybe these were Japanese titles or something. Hell, go get a gamecube and get Prime anyways...Has there ever been a bad Metroid game?
I haven't bought the GBA Metroid game, but Super Mario Advance 2 is AWESOME. Best GBA game I've played so far. I mean, it's the same game as Super Mario World, but it's such a good game, you can't go wrong with that . . .