DSi: Better sound, brighter, listen to music using the SD card, bigger screen, 12% slimmer, .3megapixel cameras, $10 worth of free points when you log on to the Nintendo DSi shop, Reset button, no fingerprints show up for there is no "gloss" covering it, a little bit bigger, plays DS games DSi enhanced games(games playable on DS but with DSi you get WAY more features) and DSi exclusive(DSi only), Records Sound(able to backmask sound too), slimmer; longer stylus, Free internet, easier to connect to wifi(WAY easier I had 1 bar and now I have 3), L and R buttons are slimmer and pop-up more, mic works better, games will be using the camera, it is slightly heavier, with full brightness battery goes out in 9-14 hours(when fully charged), Nice feel to it(like when the original DS was really uncomfortable and the DS lite came out and it felt good to use)
DS lite: is just thinner but DSi isthe better one. Also comes in a fun fushia pink. My GF's 8 year old DD has the pink one, plays all kins of games, also the kind that you take care of pets lie horses and dogs. She constantly takes pics with it and can morph them into other things and records voices and mixes up music, plus downloads free games that the kids can put themselves in the game!
Hope this helps.