If they're eventually going to phase out the real DS, it's even more desirable to get now, inho.
DS or DS Lite is my 2 cents. The GBA slot is not only part of the design of the full unit; it also has been assumed to be present by some DS software developers, meaning some DS games or other software came with something to plug into the GBA slot. Obviously, those wouldn't work completely on the bastardized DSi.
Games: I think most I like have already been mentioned, plus the entire GBA library of course. I played Metroid Fusion (GBA) on it as much as any DS game. I'm hoping Zelda: Spirit Tracks is good, unlike Phantom Hourglass, which I got to hate because of the repeating dungeon. I also liked Mario and Luigi: Partners in Time, as well as its GBA predecessor (which I keep in the same DS box, in the slot intended for a GBA cart in the Nintendo DS box made for a Nintendo-created game--am I pressing this point too hard?).