I'm the friend who preferred the GBA. The main reason I loved it was that I never had Nintendo machines as child, so I was like a kid in a candy store with portable versions of Yoshi's Island, Mario World, Mario 3, Zelda III and Super Ghouls 'n Ghosts (and Donkey Kong Country 1-3, which came free with the GB Micro I just bought). Ignoring those games, I also enjoyed far more original games on the GBA than I have on my DS. Each of the following games occupy slots in my top 50 games of all time: Astro Boy, Kuru Kuru Kururin, Advance Wars, Wario Ware, Mario Kart Super Circuit. Outside my top 50, the list of amazing games continues: Mario & Luigi Superstar Saga, Ninja Cop, Advance Wars 2, Fire Emblem, Wade Hixton's Counter Punch (dare I say I prefer it to Super Punch out without being flamed by the OP?), Denki Blocks, Final Fight One, Sonic Advance 1&2, Sega Arcade Gallery (great version of Outrun), a decent version of Shining Force (ignoring the zoomed in visual style I disliked), Mario Golf, Minish Cap (Ok so I only got an hour in before I realised I'd bought a dodgy version off Ebay and it wouldn't save)...
And probably a handful more I've forgotten. By contrast, I've never clicked with my DS. The games I've really enjoyed on it can be counted on one hand - Sonic Rush, NSMB, Professor Layton & The Curious Village....and Bangai-O were it not for the game-crushing slowdown. I didn't even like Wario Ware DS, and as previously mentioned the original is one of my favourite games ever, and The Phantom Hourglass never grabbed me (though I have no doubt it's an excellent game, along with the sequel).
So there. One vote for the GBA, but only because there wasn't the option to vote for a Game Gear with a Master System converter and magic batteries.
