Depends on the rest of your gear. The prototype I tested seems to be a hybrid, but that is the pack I'd recommend on the PCT. Sorry, that doesn't help at all.
On the PCT, given the long waterless stretches and some long resupply legs, I'd get the one with the bigger suspension. That is, the P2 replacement. When I hiked the PCT, the Zenith (predating the Fusion) worked mostly fine except for the 10 day resupply run through the Sierra and during the 2 gallon water carries in SoCal. So, you can certainly get away with something smaller. Volume needs depend on the user and their other gear. However, on the PCT you have other factors to consider (water and food).