It all depends really on what you want with an MMO, do you want something that is easy to play, has pvp, good community, high res textures, big player base....
It also depends on your age as lotro has a more mature user base and commuity whilst wow is a lot younger generally.
I've had experience of both and have been playing lotro since beta, i prefer lotro due to the story line, community and general gameplay difference from wow. Whilst it does not have proper pvp it has pvmp where you fight player controlled monsters in a specialized pvmp zone called Ettenmoors where you fight for control of keeps and gain points (imfamy / reknown).