I have both a Wii and a 360. I like them both.
But if you're a "hardcore" gamer, I'd definitely get the xbox before i got the wii (which is what I did).
The wii, with it's unique control system, is often "fun and creative", but it can also be gimmiky and frustrating, and it doesn't lend itself very well to certain types of *conventional* games, such as shooters and sports games. And if you've already dropped the $$$ on an HDTV, don't limit your gaming experience to merely 480p. Step it up to true HD.
The wii is fun, but I'd just never go that route if I was only looking to buy ONE AND ONLY ONE of the next-gen consoles. It just has to many weaknesses, from the lack of HD gaming, to the lack of online play, to the lack of a built-in entrance-way into the HD-DVD world. It's fun for what it is, but the 360 is a more well-rounded, conventional, modern console.