Easy one this, 360 all the way! Reasons...
1) A PC base unit to even rival the 360 would cost around £1,000 to £1,500 (the CPU & Graphics card alone would each cost around £400-£500 each).
2) PC tech updates so quickly so you can expect annual update costs of around £500 if you wish to keep your machine as a high end unit.
3) The 360 will last for a good 4-5 years at least.
4) The 360 is less fiddly and is easy to set up and play. PC's even now need a degree of technical know how to keep then running well.
5) The 360 has not only X-Box live but a fully updated and improved version of the greatest online gaming platform available. All games are fully integrated with live and as all boxes are live enabled (Silver level) then we should see an online community develop of the like never seen before.
6) The 360 now has HDTV gaming and can even be run over a PC monitor at the full range of PC resolutions.
7) The 360 is purely designed and optimised for gaming where as a PC is a multi task machine. A PC therefore has to be much more powerful to produce games of similar quality. Even then games have to cater for a range of PC capabilities and need to be programmed to be scalable. This means that obvious compomises have to be made with PC games where as 360 games can be made to make the most of one spec only.
Basically PC's aren't specifically designed for gaming, they are adapted for gaming. A 360 is designed purely for gaming reasons. The choice is to either buy a 360 or to buy a machine costing around 4 times as much (+ continual upgrade costs), which isn't specifically designed for the job you wish to use it for and doesn't even perform as well as the 360.
Like I said at the start, the choice is easy in my eyes.