I've tried FFXI TWICE, once when it was new and just this last summer. Both times only lasted about 2 months.

FFXI dosn't even come close to WoW in terms of gameplay. Character progression is insanely slow, it's far too heavily dependent on team-play after level 15 or so, travel is a real pain (I know it gets better with chocobos and Air-passes but still it's inferior to WoW). "Quests" are comparatively hidden compared to wow, also daily hunting guide quests are a sad substitute for the overwhelming number of quests in wow. I personally felt the atmosphere was stale (mostly depending on the cities i guess). Inventory is far too small with difficult ways to increase it and odd stacking rules. The only categories I would say it beats Wow in is graphics and the company/customer support FFXI is associated with. Possibly the ability to change jobs at will could also be a plus in its favor but when you have to go through the process of leveling them up again it becomes the same pain.

I played WoW on/off for the last five years. WoW has much more dynamic feel to the game with substantial amounts of ways to experience leveling and endgame material. Character customization is so far ahead of FFXI it's unbelievable. Colors are more vivid giving less of a listless feel while playing through its many environments. Famous and important NPCs are clearly present a well established (with few exceptions). Professions are much more rewarding and possibly less tedious. The community as a whole is much bigger giving plenty of social activities, but sadly that does mean on average there are more dicks I will admit. Blizzard unfortunately is a money hungry monster with terrible production schedules and false promises eventually making their proposed work underwhelming due to long waits and removed content. Blizzards customer support is full of bumbling fools (had my account wrongfully frozen for two months FOR REPORTING SOMEONE ELSE!) So that's usually where my off sessions from wow are. I havn't been signed up since May because of the lack of content added (until a couple days ago).

Overall though I will admit that WoW is probably an easier game and is definitely more for casual players than FFXI, but only because FFXI is so difficult and unforgiving in the way it was set up rather than actual difficulty of enemies I'm sure the people who suffered through all the most terrible parts of FFXI have a blast with some of the higher end stuff when all that painful leveling (Jobs and professions) and slow progress is out of the way.

Overall I would (maybe biasly) say that WoW is superior but FFXI is probably pretty great if you can put up with a little blandness.