It will depend who you ask. If you don't already have an HDTV why not get that? If you have a PC with an HD Monitor, couldn't you use that?
Personally I like playing console games on a nice big HDTV. I play both PC and Console games and sitting back on the couch is the way to play console games. PC you are at a desk with a mouse and keyboard. I think it is all personal preference. Some people who play a lot of shooters may steer you toward a monitor since it has less screen lag and a smaller display has advantages in some of those games as you can see everything without turning your head. It comes down to preference.