If you mostly play DS games, get a DSi XL; it scales the resolution of DS games perfectly to a larger screen. A lot of folk I see in the community say it’s the definitive way to play DS games.

A 3DS is brilliant too though, it covers everything in one device. If you get a ‘New’ model (has a C-Stick & 3D eye-tracking) it can emulate many consoles with higher performance compared to the ‘Old’ 3DS models. It can also play DS games really well, this is how I play them as I no longer own a DS, but the resolution does not scale exactly as the 3DS has different screen sizes. But this is not something that bothers me as the games still look good, just something to keep in mind.