If you like a persistent, open world, and don't like zoning into an area that is just you and your 0-7 friends, go WoW.

If you like not fighting for respawns with the retard farmer camping the area, and like competitive PvP(read: reward skill, not item grind/level) in an arena format, play Guild Wars.

Guild Wars is built more around the storylines and the elite missions/PvP.  WoW is the classic MMO that polished up what UO and Ever-Crack started, and made it open to the masses.  It depends on your taste.

No matter what, don't go WAR.  WAR blows balls.

I loved Guild Wars except for the fact that the all instanced world seemed very lonely compared to a regular MMO.   That's my only real beef with it.  I'm hoping Guild Wars 2 will be just as fun, but more persistent world based.