Could never get into WoW a few years back when I tried. Nowadays I can't go more than 5 seconds without commenting on how garbage the game looks (it has NOT aged well).
To me WoW covers the other spectrum that FFXI is unable to fulfill, as far as my fun factor in MMOs go.
I have sticked with FFXI this long because it is overall the general background, and thus the story, environments, lore, soundtrack, playable classes, races, etc. have captivated my interest and imagination better than any other.
On the other side though, WoW fulfills the simplicity and direct involvement and social aspect I couldn't get out of FFXI. In WoW, I actually enjoy progressing and advancing my character in both level and equipment as well as passive skills such as crafts and whatnot, and tinkering with the skill tree to master my playstyle, while FFXI does not allow for such headroom, where as it is pretty much comes down to using the right gear, or using the wrong gear.
In short, FFXI covers the backdrop and foundation of a great RPG (online or off) that kept me with it for so long, and WoW covers the MMO elements that are somewhat void in FFXI.