As Armin says, the main difference is that there is a switch for the P to switch between swing and tilt which are controlled by the same knob, and the P2 uses 2 separate knobs, eliminating the need for the switch.
I've handled a couple of P's and a P2 now. Someone who knows more about them than I do, says that my P has in fact been ugraded with some P2 era parts, which are in many cases interchangeable with the older parts. A P2 will be newer of course, so it may be in better adjustment than many older P's. A P that has had many years of hard use in a studio, may need realignment and some of the nylon gearing may need replacement, but some studios send their cameras for routine maintenance, so if you can try the camera before purchasing (or budget a few hundred dollars for maintenance if you're purchasing by mail or eBay), that's best. The most likely problems you will encounter with an old P, I sense, are dry level vials (about $50 each to replace yourself), misalignment/wear (more expensive to fix), and possibly just loose parts, which you can tighten up yourself. You can also adjust the levels yourself, if they've come out of alignment, which is common.