Xbox 360 is hell of a lot more comfortable for me on longer play runs, not due to sitting (as my spots are pretty comfy, actually), but because of the stress on my wrist from the mouse, even when using more wrist-friendly applications and such, it still causes me great annoyance.
Also, why is the OP trying to claim that PC gaming is the "better" experience? I play PC and 360 and love both, only find 360 more comfortable for my wrists, why can't people ever just get along with the whole console vs. personal computer crap?