My answer to the same question about a year back:
The internet- IT SUCKS. As simple as that. Some sites may be good. But others... well it's pretty bad.
Battery life- 14 hours {DSi} vs. 19 hours {DS Lite}. You choose.
GBA games- I've read a few reviews mentioning being able to download SOME GBA games. It can't play the cartridge, though. Sorry.
Music- NO MP3s! That sucks, don't you agree? {Heh. I rhymed.}
Pictures- Hell, I have a cellphone AND a camera for that.
DSiWare- How much are you ACTUALLY going to use it?
The DSi has a few good features, such as you can get out of game by pressing the internal power button. You can remove the game cartridge on the main menu and add a new one, if you feel like it.
It all depends what you want, though.