Well, nds tend to be cheap, and there's ton of games, even when you remove the shovelware from the equation, and even some of the shovelware is a fun, if mindless time.
With that said, if you can a 3ds is probably better. To be fair, the 3ds library isn't even close as big as the ds, but it is retro-compatible so you still have full access to the ds library, if hacked you can also run gba roms natively, and I think there's hombrewed emulators for nes, gb, gbc and snes. Talking about hacking, once you do that with the 3ds you don't need flashcarts to use roms.
Downside is that 3ds is significantly more expensive. The ds is the most sold console ever so there's a shitload of them in the wild driving down the price.
Anyway, if games on the level the ds can do are good for you, then yeah, it's worth it. There's also influence in what kind of games you like, for example, its a good console for jrpgs, but terrible for fighting games.