Sorta depends on whether you have any GBA games that you play. I've only got KH: CoM and FFTA, but I've also got a GBA (so I can play original Pokemon). Ergo, I don't need a DS/DS Lite, but a DSi could be useful.

ESPECIALLY for FFIII Mognet. My wifi doesn't work with DS/DS Lite, but it would with a DSi. If my current DS Lite conks it, I'm getting a DSi on that basis, and on the fact that I've got a working GBA.

Don't factor cost, $40 is not much when there are some major differences.