I have a DS Lite, and I am rather happy with it. Though, I have tried the DSi, and it's something worth owning if you like messing around with the camera it has, even if the quality isn't that great. Another advantage the DSi have besides the camera is that you can download games from the DSiware just like how you can download games from Wiiware, and some of the games do use the camera feature, so I'd say its worth having. Just don't try browsing the internet on the DSi, even if it can do so.

Edit: Oh yeah, the DSi doesn't have GBA support, so that's a big loss if you enjoy playing GBA games.