I'd vote for DS. It has better games than Wii (so far). I would personally get a DS-lite instead of DSi. A used DS-lite can be had for $99.
I have really mixed feelings about Wii.
If you have family members who would play with you regularly then Wii might be a better idea. Just keep in mind that after all the add-ons (and extra controllers for 3 other members) it costs more than the other two. That said, local multiplayer on Wii IS a lot of fun Mario Kart, Super smash brothers, Wii sports all are pretty good. Unfortunately those are the "only" ones worth mentioning IMHO. Only a few of the party games are as good (super monkey ball and raving rabbits (sp?) had good reviews.
If you are a fan of older gen games, Wii Virtual console is a good option however, unlike XBLA or PSN these are straight emulation games and most games look and play dated. That said, it does have some great out-of-print gems like super mario RPG, mario etc.
