All these games are so very addictive, I don't think you'll find anything different with GW or FF then you did with Wow. I spent over 4 years with FFXI to only recently go into WoW myself (did some GW in the middle of that). I understand your pain with the addiction - as you say, everything in moderation.
I wouldn't recommend FFXI if you didn't like the time and pace needed to get a character to max in wow (in playing both, i find the lvling system in wow to be much easier/faster). FFXI is much longer grinding process and pretty much after level 10 you can't solo (unless you are a beastmaster or summoner and it's still quite hard as it wasn't meant to solo).
FFXI has great story, better then WoW imo, because it has a huge storyline that is continuous throughout the missions and the quests. FFXI is a beautiful game, even if it is 7 years old, it has many unique things to it if you don't just want to obtain lvl 75. But I like wow better because the solo/duoing ability and the fact you gain xp from questing. In FFXI you gain xp by killing mob after mob after mob... and this relies that you have the 'perfect' 6 person party set up. If you hate level grinding in wow, you will toss FFXI out the window in irritation in less then a week. ^.^ But since there is SO MUCH to do in FFXI, you may wanna give it a spin if you are an FF fan at all - I don't regret my years of playing in that world (and I never played any FF before that one).
Guildwars I think is pretty fun and the nightfall expansion is very nice. I just found I couldn't get into it as much as I did on FF and WoW. The story seemed to be lacking for me. The complexity of the control system wasn't an easy transition for me from FF at all. I really liked FFXI's control system, so that might be my stubbornness (I am very stubborn). The one thing I didn't enjoy about GW is to finish the really cool mission storylines it required large amounts of people (more then FF did). I also thought the graphics were kinda blah next to FFXI and WoW (a bit grainy). However GW has great holiday and world events I found to be superb next to FF and WoW events. I didn't play GW as long as the other two, so my opinion could be totally off. I do know that with the nightfall expansion that soloing is actually a possibility for most of the game on the regular non-mission quests. I did enjoy what I played with GW, but I haven't even tried to go back since joining WoW (I'm only about 4 months in).
Overall I think if you can try both GW and FFXI, you should, that way you can find a MMO that really suits your wants in a game and how you like your playstyle to be (as in do you like the party scene, soloing mostly, questing, very good storylines or animation, gear/character looks, etc).
Most of all... I think you'll find addiction in all of them so if that is your biggest thing - best advice might be to stay away from allllllll of them.
Don't know if I helped at all but... I always enjoy this topic ^.~