I have and still kind of play both of WoW and FFXI. I beta'd and played FFXI since first NA release date, and got everything I wanted out of it. It is very party based, forcing you make more friends. I knew about everyone on my friends list on a first name bases and would BS with most of them on a daily bases.
On WoW, I know about 3 out of 10 people who are on my friends list and do stuff with them rarely, cause well, I dont have to.
I see WoW dying first cause after this last expansion, they are shotting themselves in the foot constantly. A Hero class that basically dwarves all other DPS, completely ruined PvP, content that was murdered within the first month of launch. It just seems BLizz is getting tired of trying to keep WoW interesting.
Though unless you are an end game whore in FFXI, there is no causal play. I pay to talk to friends on there once or twice a month.