If you want to play GBA games, buy a Gameboy Advance Trust me, they GBA games are a lot more 'this feels right' on a Gameboy Advance then on a DS. The most off-putting thing about GBA games on a DS is that one screen is just black. It feels wierd.

I have both a PSP and a Nintendo DS and if I was recommending just one it would be a DS, easily. For the games alone, like:

Phoneix Wright, Trauma Centre, Animal Crossing, Mario Kart, 42 All Time Classics, Lost in Blue, Castlevania Dawn of Sorrow, etc, etc...

Too many great games already on the Nintendo DS.

But for sheer potential and 'what might be released in the future' it would be the PSP. There are some great games already out for it (Tears of Vermillion, Lumines, Guilty Gear) and there are some good looking games already in development.

Of course, I have both, so I don't care which is better, but for recommendation it would be the DS.