I think its a mixed bag. The DS has some of the most rabid support for any Nintendo system since the NES. But many of the games are rather lackluster and getting to the 'good' games is hard. GBA had this problem here and there, but more games had a higher ratio of quality.
I don't think I could compare them yet however. The GBA actually started rather weak, with a rather limited selection of good games. The DS within its first year had some of the best (or most fun) games in its life so far, like WarioWare Touched!, Kirby's Canvas Curse, Osu! Tatakae! Ouendan! and Jump Super Stars. Yet while the DS started out so strong, it had a year or two where it seemed only Nintendo was supporting it. Kind of like the Wii was struggling with. GBA never had this problem, because once support for the GBA came, there was good games consistently from that point on.
I think we'll have a much better view of how the DS compares to the GBA near the end of its life....or if a new 2D Metroid comes out as good or better than Zero Mission. If that happens, DS wins hands down.
Six upcoming games you should look into:
