I've been playing Everquest II on my PC for the past week or so. It's a great game that can really draw you in. But wait! What about poor Mr Xbox?! Have I left it to gather dust?
Nah. EQ2 is great because of its depth. There is always more "just around the corner", and you get drawn in to find out just what that is. But it isn't an adrenaline-raising game like Ninja Gaiden or Burnout 3. So for certain moods, the Xbox is going to be the winner.
But what about the times you have that "deep game" mood. Has the Xbox lost to MMORPGs on the PC? It very much depends on your play style. I'm generally a solo player. I want to go explore and to kick some ass. The MMORPG is a continually evolving system, so there is always more. I'm not there to interact with other players, to group up, and even less for PK (tho EQ2 does not support PK which is great). And we have seen a deep RPG game on the Xbox before: Morrowind. I played that sucker till my eyes bled. EQ2 is just another deep RPG for me (tho, to be honest, it is also great to play a separate character with my buddy; co-op RPGs rock).
Social players might think, "but the Xbox doesn't have a social/co-op Morrowind, so I'm gonna have to go to the PC." Yes, that is generally true for now. I think it is going to change in the next year or two, however. I'll also point out that a game like Halo 2 has changed the entire notion of social play. The support for clans, bungie.net, big deathmatching, different game types, etc etc. It's a whole new game world. Not an RPG and it lacks that kind of depth, granted, but it provides a great vehicle for social gaming.
I'm looking forward to the next Elder Scrolls (the official name of the series that Morrowind is part of). It is being called Elder Scrolls: Oblivion right now and is being targeted at the next-gen Xbox console. Will it support Live? I'm guessing it will be -enabled, but there won't be any real co-op or multiplayer support. Bethesda is pretty hard core into the single player experience. And I love 'em for that. It does mean that a co-op RPG is going to have to come from somewhere else. Not sure where right now...