I had a P2 inflated on me at a show a couple of years ago now. It was an adult medium on it's loosest setting, and I'm anywhere between a size 8-12 on top depending on brand. I'm hardly big. When it went off, it went off with such a loud bang it honestly made me jump and scream... I thought a gun had been fired. I was also quite badly winded and struggled for breath for a while after and had to quickly rip it off.
Another point, is the canister connection on the P2 is highly complained about as it's fine plastic, so fractures easily and only a bayonet fastening (so higher risk of it coming loose). If you like that particular style, look at the helite, the company who made the original P2.
I have a hit air. My reasoning behind this - it inflates outwards, not inwards, so it's worn closer to your body (so less of a risk of getting caught on a low branch out hacking, and flaps less!) and looks neater. It tucks up away from the cantle and your neck, so allows freedom of movement until inflated. When inflated (and by George have I tested it!) you get greater protection around your body than with the P2 - it has better neck protection, coccyx (as it's press studded up, it can be longer on inflation), spine and ribs. I'm perfectly comfortable when inflated. It's also a lot quieter on inflation. I have it on video somewhere I'll try to find mine inflating. Unfortunately dad was a little shocked at my fall so it wasn't fully all recorded on this video I have.