Let's face it, the question really leaves a lot of loopholes for one being better than the other. What's the spec of the PC in question? What kind of games does the person like to play? What kind of PC? To ask a few...
PC --
X360
greater cost -- less cost
backwards compatible -- backwards compatible w/ most games
keyboard + mouse -- wireless controllers
upgradeable -- not upgradeable (for the most part)
video usually on monitor -- video usually on TV
internet capable -- internet capable
Instant messenger\email -- instant messenger/email through MSN
voice communication possible -- voice communication possible
part goes bad\easy to replace -- part goes bad\send in to Microsoft
lots of games -- lots of games
Let's face it, they both have pros and cons depending on the person they're intended for. I'd say I like to prefer playing on my PC for certain types of games (FPS, RTS) but prefer my X360 for other types (RPG, Sports)....however I do enjoy some FPS game son X360 such as Halo3, Bioshock, and Gears of War while also enjoying RPG type games on my PC. If I couldn't have both I would probably side with my PC based upon functionality and the ability to upgrade over time. However, since I have both i will continue to bathe in the luxury of playing each as I see fit.