I prefer the ds,
gba was all based around the idea that it was a 32-bit portable so a lot of developers were trying to make it seem like a portable playstation
so the gba library was plagued by pseudo 3D games ( mostly platformers ) with terrible gameplay...
even with awesome games like the metroids, DK, and the kirbys, it wasn't anything special, the gb and gbc had more memorable games than the gba
also there's those poor mario ports ( and zelda alttp ) with washed out colors and tweaked gameplay and level designs to make it easier for casual players, at the time they were with some stupid idea that portable games needed to be shorter and easier
that idea even carried to the first few years of the ds, I remember around 2007 someone interviewed the guy responsible for ninja gaiden ds and he mentioned that he wanted to make the game 30+ hours long
but the company pressed him to make it shorter telling him he "didn't understand" portable gaming
not to mention the original gba screen was so bad you could barely see anything, and the first gba sp had a really crappy backlit screen, gba sp brighter was the only one with a decent screen
and the DS, I don't like the touch screen concept ( I really don't like scratching the screen and having calibration issues ), but at least it got plenty of awesome games, a lot of them not using the touch gimmick
the first 2 years or so were pretty awkward for it, with people trying to figure what to use the lower screen and the touch for. but things got pretty good after that
we were even lucky enough to get all dragon quest games localized ( the gba had only 1 dragon quest game in japan, a so-so DQ7 spin-off )
and a zelda game that even if it has lame touch controls it captured perfectly the whole feel of majora's mask ( of course i'm not talking about philler hourglass )
for some reason nintendo was awkward the whole time with the gba, it's like they were ashamed of making games
the commercials barely even showed the games at times, magazine ads were really random and weird
they even wanted to change its name to City Boy and they made the gba micro so that "you won't be embarassed to carry it around and play it at parties" ( actual quote from a official nintendo magazine )
I think most people that pick gba it will be out of nostalgia, and not because of the gba itself