If you have an SP, get the DSi. If you don't have an SP get the lite. Unless you don't play GBA games at all. It's that simple. Because the original GBA doesn't have a light up screen, and the DS's and SP do. If you don't play GBA games there's no point in buying a DS Lite. If you do, but have an SP, I can't see a reason to get a DS Lite unless you don't want to carry 2 systems around. I do anyway cause I still play gameboy color, and I have a Lite.
Bottom line is I'd chose a Lite because it is more compatible, but if GBA is not a concern, then unless you don't have the money go with the DSi. But I'm not planning on ever cashing in my Lite. kthx