I've been going to AO since its second year, so this year was my 6th. It's an amazing con ^_^
However, fandemonium is in a completely different vein. While Jeremy (AO) and Borneo (Fandemonium/Fan) work very closely with each other, they are quite different. AO is primarily and quite insisitently anime, games, and Japanese culture centered. Fan, on the other hand, is an all around fan convention. It is sci fi, fantasy, gaming, anime, etc.
If you are a big cosplayer, Fan is not your venue. The level of cosplay is, er, not so high >< What I mean is that cosplay is not a main focus of the convention. It's all about the gathering of nerds.
I kind of enjoy fan, but i typically go because it's only about a 45 minute drive for me. But, to be honest, I wouldn't drive 6 hours for it. If you want to go to an Idaho con, there are 2 in boise, and 1 in nampa.
In November (over thanksgiving weekend)we will have Tomodachi Fest for its first year, and then AO 8 in March. I would recommend one of those over Fan.