I have owned a HVX for about 3 hours now, courtesy of Tommy Mack and EVS. Thanks Tommy.
I've never used a P2 before, but I can give you the basic concept.
A P2 stores a little bit of storage (8g at most), and a firestore stores a bunch (100g).
If you are doing simply controlled shots (meaning like, a short film, or a commercial), you can get away with the P2, and your workflow will look like this.
1. Shoot for a few minutes.
2. Take card out.
3. Put card in something else.
4. Copy info from card to something else (hard drive, whatever)
5. Erase Card
6. Put Card back in Camera
7. Step 1, rinse repeat.
Which, in techno-speak, blows.
The Firestore workflow is ALMOST exactly the same except step 1
1. Shoot for a couple of HOURS.
There are other ancillary concerns here of course, with -two- P2s, and a person offloading them for you, you could shoot more or less indefinitely, and only have 30 headaches a day organizing, and keeping things going.
Similarly, with the firestore, you have to worry about its battery capacity, as well as the fact that you have this physical thing to attach to either the camera or yourself.
But, in my imagination, there's no reason to get a P2 card.
The flip side of this, is that you get one free, anyway, if you buy an HVX between now and march whatever, so if money is no object, get a firestore. If money is an object, be prepared to limit yourself to 5-10-15 minutes of uninterrupted shooting.