Why not just enjoy both? Play Xbox exclusives on the console (Fable 2, Halo 3 etc) and play PC exclusives on the PC (Counter strike etc) like me. You don't HAVE to choose one over the other, like the machine has feelings or something! Both have their own separate advantages and it's up to you to take advantage of each and enjoy them both.
If you're talking to me it's down to the amount of time I'd get to play them that's why I'm not buying both.