First order of business, do NOT get the original model DS.
Good, now that that's out of the way... you really can't go wrong. The DSi has a slightly bigger screen than the DS Lite, and the DSi XL of course has a WAY bigger screen, but aside from that, the differences are negligible. You'll NEVER use the built-in cameras, and the online store is extremely useless. I've owned every model, and they're all great. I'd say, decide what size you want. If you want big (but will still fit in an adult pocket), get the XL. If you want small and light, go for the Lite. The regular DSi is not worth the added cost over the Lite.