I thought βόλεϊ was volleyball too. What do most people normally use for volleyball?
I believe βόλεϊ is the more commonly-used, everyday, λαϊκός word.
πετοσφαίριση is the more formal, purist formulation. I think someone even said elsewhere that if you use terms like πετοσφαίριση instead of βόλεϊ, μπάσκετ, etc, you'll pretty quickly mark yourself out as a foreigner.