well, i play on both because of the games that are available for both systems. obviously, there are exclusives to the console world (a la Force Unleashed and Forza), but there are also a few exclusives to the PC market as well ( a la Crysis). aside from that, though, there are also some games that play better on a console than a PC and visa versa which is also a factor in which system i game on.
currently, i've been using an Xbox 360 on my new 24" monitor, and it works just fine for what i've been playing. however, i'll also get on my computer and to stretch its capabilities in games like Crysis and World in Conflict.
in my mind, the strengths of both systems ensure that most gamers will benefit from owning both a nice PC and a console. if i had to pick one over the other, though, i'd have to go for the multifunctionality of a PC over a console. it may not be the best gaming platform in every situation, but since it lets me do so much other things, i'd have to pick it.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind