I think that one of the greatest things about being a vampire is having a special ability therefore I am turned in making a desicion.
The good thing about the Volturi is that they will train you ro immideatly perfect your abilty yet the cullens will take care of you and truly care. I guess being a vampire you have eternity to learn =)
I'd pick A. Because I don't want to drink human blood. And, the Volturi are into all this war stuff, and that's just not me. Also, I'd rather learn to perfect my gift on my own, I like feeling independent. :D But that's just my opinion. And if the Volturi were a little more peaceful, I might pick B....or i'd be like Eleazar and just leave. Haha. I'm not sure.
I would choose A because it seems that Volturi don't really seem to believe in freedom of choice. Especially if you end up having a really cool power. If you choose A you can stay with them or not or go join the Volturi later or do whatever you want.