I love them both. Objectively speaking, the NDS had a better, more varied library of games.
However... I'm a sucker for Game Boys. There is something about their design, simplicity and the kind of games you could play there that really fascinates me. Sure, there is nostalgia involved. But... there's more. Also, while I know NDS (and Lite models) are backwards compatible with GBA... GBA is backwards compatible with Game Boy and Game Boy Color. I love being able to enjoy my modest collection of the entire Game Boy family on a single device!
Let's just say this: I'm 29 years old and to this day, I still end up coming back to charge my GBA SP (I put a IPS screen + bigger battery on it) and play some Golden Sun, Pokémon, Tetris, Wario Land, Metroid or Zelda (among others). I can't say the same for the NDS (though I have at some points come back to it too).
So... yeah. GBA for me.
