I'd say go for the GBA - just over half the price, and the games seem more compelling. This holiday season there's a new Zelda, GTA, Kingdom Hearts, Final Fantasy I + II, Boktai II, Kirby and the Amazing Mirror, a new F-Zero, Donkey Kong Country 2, Metal Slug Advance and that's just the start of it. Of course, there's some gems already out there that you should get stuck into if you haven't yet, such as the Golden Sun series, the Pokemon series and the Advance Wars series.
Compare this to the DS, with about 5 games out and the flagship launch title being a rehash of Super Mario 64, I'd say go for a GBA.
Looks like Nintendo's biggest competition to the DS isn't the PSP, but the GBA.