I agree that ultimately it is up to the seal which consists of the seal components, their mating surfaces and their exposure and resistance to compromise from foreign particle contamination. Another consideration is the number of times the seals are subjected to movement in a hostile environment. The fewer times a light is cycled on and off the fewer times the seal is subjected to compromise in dynamic action VS static forces. If I were going to use either the PD or C pak in wet and dirty or submersed environment, I think I would prefer to use the twisty activation of the light over the tail activation. This puts the "action" at the bore seal between head and pak and this is an area that is easy to inspect before and after the fact. One can do a BS (blow/ suck) test on the pak prior to use and visually inspect the head to pak seal as well as feel its engagement when putting the light together.
Hmm, I dunno what to say, but my c-pack has been very good waterproof. I certainly cant't blow of the cap, and sucking air into it is out of the question. Even worse: when I use primairies for longer times, the rubber boot will blow up a bit, but there is no leak. I've been swimming with the c-pack for several times, all together I guess over 4 or 5 hours. Never had a leak.
The o-rings in the c aren't moving, opposed to the PD's. The PD-sealing ring is moved a bit with every click. But I also must admid I regularly take the c-pack apart, clean and use a bit of Krytox 260 (less liquid as the 50/50 from the shoppe) to lube the inside ring a bit. I guess this helps a bit as well for keeping waterresistant in more or less shallow water.
But hey, I'm a c-pack fan and don't have a PD. yet...
Even when I drop it into cold water after it has been warmed in the sun, I can see the boot retract a bit, but no water is passing the o-ring at the head. I can see water until the o-ring, even a small bit in the groove of the oring, but it isn't passing the o-ring dispite the clear under-pressure (the boot is sucked/pushed inwards a bit!)
I also forgot to mention I replaced the stock o-rings with Vitons that are a tiny bit less smooth and could give a more thight fit!?