I've been playing WoW for the past week or so and have gotten to lvl 23 with a human mage. My computer is far from being top of the line so whenever I play either game, my graphics settings are dumbed down to the minimum. But as far as I've been able to tell, wow graphics are actually pretty good. The only knock is the cartoonish avatars.
Wow and FFxi cater to different types of people. Wow is definitely for the casual player who doesn't want to devote their life to playing a video game yet still have a character that is is respectable in ability. FFxi is all about the power gamer who has hours to camp/exp.
I think the aspect of wow that appeals to me is the fact that I don't have to search for a balanced party to get experience at a reasonable rate. In wow everyone talks about how you exp mostly by doing quests, but you need to remember that a lot of the quests require you to kill a certain amount of monsters so its not exactly just running from one npc to another.
Playing ffxi, spending so much time trying to find a party to lvl my thief burned me out. The biggest complaint I have with ffxi is how the various classes aren't balanced because it directly affects how often you can actually play. Also, melee jobs outnumber casting jobs by far creating a larger demand for certain jobs while pushing other jobs to the back of the line.
Sure, people playing wow bitch and complain about how the various classes like paladin are not balanced but it only affects one aspect of the game. Just because I think the pld class in wow is overpowered won't change how I play my mage except maybe I'll avoid pvp (which I don't really care about). In FFXI, or any other mmorpg, the biggest appeal imo is exping to go up in levels so that you can explore, etc. The job issues heavily affect a person's enjoyment and play.
In terms of endgame content, ffxi definitely has more events potentially available while wow provides more flexibility at the lower levels.
Anyway, thats my take on the two games. Wow vs ffxi isnt a coke vs pepsi issue. I can't think of a snazzy analogy, but the two games cater to different groups of people.