I have a hit air.
1. The coverage for protection is greater than the P2/helite (they're similar designs - helite are the original P2, before P2 went off alone) - it immobilises your neck better, actually covers your coccyx and ribs (unlike P2)
2. The design is neater - the hit air uses poppers to remove excess material when deflated, so folds up away from the cantle and away from your neck/shoulders
3. The hit air explodes rather than implodes - this means the jacket sits closer to your body, therefore flaps less when deflated (I also hack in woodland, so was worried about baggy P2 getting caught up in trees), doesn't hurt on inflation (unlike P2 - I'm a ladies size 8/10 on top and found the adult medium in P2 the right length, had it on the baggiest setting and yet when inflated it felt like my ribs were being crushed)
4. The hit air is quieter than P2
5. The P2 has a bayonet fastening for canisters and from reading the net, this is a weak point unlike the helite and hit air
6. Personally, I think with the P2/helite you need to wear a BP underneath to make it even remotely comfy on inflation
7. Hit airs were initially designed for motorcyclists - more testing behind them
8. The hit air is cheaper!
I've fallen off 3 times in my hit air now, twice SJ and once XC. Yes, I did have an argument with a palisade, and no I didn't ache one bit the next day. I took the jacket off, hopped straight back on and carried on.