I own both a GBA and a DS. I own about 4 GBA games (I've borrowed/played through about 4 others) as well as about 4 GB/GBC games. I only own Mario for the DS, so this post will mainly be regarding the future lineup of the DS.
Is it just me or does the DS already have a better lineup than the GBA (not necessarily in numbers, it's only 6 months old, afterall)? The GBA, despite a few good original games, was so full of ports it was quite crazy. But already with the DS we have a new Mario platformer, a sequel to Mario and Luigi within the first year, a Metroid FPS, a new Final Fantasy, brand new games and franchises, online titles, etc.
How do you feel the two stack up? I think the DS is by far the best handheld available, as far as games/future releases go.