The 360 is easily the more well-rounded of the two systems, and 2007 for it looks like arguably one of the greatest years for a game console ever.
I was lucky enough to get a Wii, and now I'm wondering if there are even going to be five games worth paying full price for next year. I'm guessing (after Wario Ware) Metroid Prime is a lock, and Super Mario Galaxy will probably be out by Fall (anyone thinking it's really hitting within six months of launch is laughably delusional), but that may be it. My sense is that Super Smash Bros. Brawl is 50-50 at best for 2007, and any other AAA Nintendo title you can think of is probably a ways off.
Unless you're a legitimate Nintendo fanboy ('out' or not), the Wii is really the ideal second console, the counterpoint to your 'grown up' 'next-gen' console of choice. Sure, you can get more to start off with if you set aside $500 for either, but I just feel like it's going to take another 12-18 months for it to really develop a worthwhile library (outside of the Virtual Console, anyway). And no, kids, my pessimism doesn't mean I'm selling mine. Nice try, though.
EDIT: Another major concern I have for the Wii is how many third-party developers will even bother with its version of multi-platform games once the current/last gen systems are all but completely dead and buried (like, oh, mid-next year). I'm not sure exactly how this all works, but it's one thing for them to tweak a game for the PS2/Xbox/Gamecube and slap it on the Wii with a different control scheme, but it's got to be another to make a graphically dumbed-down version of a 360/PS3 game for just one console. Is the Wii library going to be even smaller than the 'cube's? Logic, in the long run, would seem to dictate "Yes."