I played more GBA games on my DS than on my Game Boy Advance.
But I also did play more GBC games on my Game Boy Advance than actual GBA games.

The DS is my favorite system, but when I want to retro play handheld games my modified GBA get more use.

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.

Both are great systems.  However the GBA was the last device truly dedicated to 2D gaming.  Astro Boy, the Castlevanias, Final Fantasy Tactics advance, etc....  The GBA is where it's at.  Plus you can use your Gamecube to play the games on a TV set.  If I could do something like that with the DS, I might be changing my tune.

FFTA is such a gem. I hope they add it to NSO at some point ^^

It's an unfair assessment but I grew up with the GBA and some of my most fondest memories were made while playing games like Mario Tennis Power Tout, the SNES ports of Super Mario platformers, the Dragonball Legacy of Goku Trilogy(I'm weird lol) and Pokémon games which still has the undisputed best Pokémon game in the form of Emerald !

I did experience some pleasure of the DS, where I began delving in RPGs a bit more but I didn't own one so I can't give it the advantage.

But in hindsight playing those games on my 3DS, it's just as robust as the GBA library, if not more in 3rd party cases.

Honestly, I'll give it to both !

You're not weird. The Legacy of Goku games were great ^^

Nintendo DS Lite to be exact I would choose.

Aside from being backwards compatible with GBA games, but the DS had some really amazing games with a ton of 3rd party support.

The incorporation of the dual screen was a fresh take and was utilized very well with many games.

The DS truly deserved the massive sales it had as the hardware was nothing short of a masterpiece.

The DS. While I still liked the GBA for its content, I wasn’t crazy about the hardware itself. As soon as I had a DS I never used GBA hardware ever again. My major problems with the hardware were the Dpad and the lack of backlighting. The Dpad, IMO, felt off compared to other handhelds, I remember my fingers slipping a lot during action games like Castlevania Circle of the Moon. Granted, these issues were mostly fixed with the SP, but too little too late for early adopters like myself, as we were already aware of what would become the DS by that time.

Content, while the GBA did a good job, the DS (IMO) was my all time favourite for handhelds (Switch excluded), having not only a much larger library of great games, but a significantly higher volume of great games. DS was also significantly more inventive and had a much more rounded library genre-wise. Phoenix Wright, Hotel Dusk, Another Code, updated Dragon Quest 4 and 5, plus Dragon Quest 9, NSMB, Mario Kart DS, the Brain Age games, Animal Crossing New Lead, Elite Beat Agents, Professor Layton, and a ton of RPGs, including Chrono Trigger and 999.

I think what draws me more to the DS that it was more revolutionary than the GBA. The GBA experience was more about reliving old experiences (from 5-15 years earlier) on handheld, the DS experience was much more current and fresh. I wanted to talk about the 3DS as well, which is probably my second favourite handheld console of all time and while the 3DS wasn't as successful, I found the 3DS continued that fresh feeling/current DS experience. The 3DS commercial success was hit hard by two things: competition from mobile and a strong (irrational and nonfactual) fear campaign against glasses-free 3D.

I'm going to take the time to break this down.

• Hardware (DS) - Overall, I'd say the DS hardware was much more interesting, while still providing classic style controls comparable to the SNES. The GBA loses points for its lackluster first iteration and a surprising lack of X and Y buttons, though the GBA SP is admittedly great.
• Action-Advenrture (GBA) - The GBA is home to a couple of the best Zeltroid-style Castlevanias, two great Metroid titles, Legend of Zelda: Minish Cap, Mega Man Zero, etc. The DS had its own great Action-Adventure titles, but to a lesser extent.
• Adventure (DS) - Games like Ace Attorney, Professor Layton, and 999 were natural fits to the DS, thanks in large part to its touchscreen.
• Casual (DS) - Obviously the DS wins this category.
• Platformers (Tie) - Both platforms had a number of strong contendors, largely from similar series. I'd say that the GBA had the edge if we count retro ports/remakes equally to new titles, the DS wins otherwise.
• Pokemon (DS) - Yes, I feel that on Nintendo handhelds this should be its own category. And although I replay Emerald more than the DS games, the DS was arguably the best Pokemon platform ever, with twice as many core entries, the best remakes in series history, and more/better spinoffs like the Ranger, Conquest, and Mystery Dungeon games.
• Puzzle (DS) - Both had less big puzzle games than I remembered, and largely have similar series being represented. I picked the DS because a combination of Tetris DS and touchscreen controls.
• Retro Ports/Remakes (GBA) - The GBA had 100+ games from platforms like the NES and SNES, not to mention a fair number of compilations. In contrast, the DS had a lot less of these, possibly in part due to the lack of analog controls making N64 ports/remakes inadvisable.
• RPG's (DS) - Both platforms are home to some of the greatest RPG lineups of all time, and in some ways the GBA outshines its DS counterparts, but I have to narrowly give the edge to the DS. The sheer number of games new and old on the DS was remarkable.

Overall: The DS

GBA was more like an evolutionary step from the Gameboy/Gameboy Colour than a massive leap forward.
Basically was the SNES compared to the NES. It was great, but not revolutionary.

The Gameboy Advance was also only on the market for 3 years before the Nintendo DS arrived, so it never really had a chance to truly shine, but despite that, it still managed to shift 80~ million hardware units and had some extremely enjoyable games like Metroid.

The DS was a banging system, it brought something fresh and new with it's hardware... And is the number 2 best selling gaming system of all time.