For the longest time I have voiced my disdain towards the DSi and the newer DSiXL. The biggest problem with them is that they CAN'T PLAY GBA GAMES. No backwards compatibility is an instant no-no for me.
That being said, my friend bought a DSiXL recently. I must say, the improved resolution and bigger screen size make it seem like an ACTUAL upgrade. Everything else is moot, the cameras, the music player, internet... Who cares? It's a GAME system. But the XL gives the actual GAMES a boost. A small one, but a boost nonetheless.
If you really want a higher resolution and slightly better looking games (not by much mind you), I would reccomend the DSi XL. Otherwise, get a DS lite. It's more compact than a DS and has a better backlit screen.