Technically, the Xbox 360 is better, and the way the games have been made for it will also be superior (except with ports i guess) however, given a few months PCs will be easily as fast as the 360.
Also for anyone willing to compare prices, just think that you need a £2000 HDTV to get any benifit of it over having a standard Xbox... unless you like playing in 640x480. Either way, the price of an equivilent PC should be able to compete quite nicely.
The main technical aspects the 360 wins on are it's 4x FSAA with little to no fps loss, that's certainly a big aspect of it and I believe it uses unified shaders, which is something that PCs will only get with Vista and WGF1. Also despite the fact that I hate PPC chips, it does have 3 of them in there, and the games should be coded to take advantage of them.