You'll probably want to refer to this thread:
http://www.tfproject.org/tfp/showthr...&highlight=GBA
It's about the best GBA games.
I'd get a Gamecube if you don't care about portability. I love my GBA, but if I had to choose, and didn't care about portability, then I'd stick with the Gamecube.
If you truly want a portable system, then you can't go wrong with a GBA. It's definitely worth the $99. They're so damn cool.
The Gamecube is certainly geared towards younger kids in certain genres, but that doesn't mean the adult-oriented games suck. The Resident Evil titles are incredible, and I personally think Eternal Darkness is worth buying the system for.
-Lasereth