I miss my TurboGrafX and TurboExpress... Those were truly awesome systems, 16-bit, only had to buy one game card, and you could run it on both machines. It's too bad that nobody else opted to follow that line of thinking.
If you don't mind an older system without all the uber famous games, the T-Express would be a decent alternative, and should be cheaper than a GBA. Some of the games for it still rock and I can't recall having seen comparative games since (I'm thinking Military Madness, and games of that ilk).