Well, from playing/completing GW with severla chars (all 3 chapters) then being bought WoW...
If you want to pickup a casual MMO that you can go on when you feel and when you don't - GW every time.
The battles/missions are good, some good quests for prophecies towards the end.
Hard to find good players, but they are there.
If you want a fully immersive MMO - WoW.
It will take over your life.
Time will seem to stop.
But it's darned good fun.
Again, the guild makes the difference, average player age on the horde side seems to be higher than the average age of a GW player - And it is easier to find decent players I've found.
Put it this way - Completed all of GW using henchies/heros (hero's for NF).
Got asked to help someone through the last mission in Nightfall - They died twice.
(usually no-one died)
So in general... Bots > Players in GW.
Unless you get a good guild.
(Basically it's easier to 'solo' missions than it is to find people to help - Whereas in WoW, you NEED players to complete most of the interesting stuff)
In WoW, until you find a good guild - Your friends list will fill of capable players much quicker
Not played the rest, know peeps who have tried LOTR, loved the beta, didn't like it much post-beta.