I agree, it doesn't really matter... I believe the best piston will outperform the best diaphragm but my guess is the only way you would notice is to hook them up to a work-of-breathing tester. If you get a decent balanced 1st and 2nd stage, you're going to be a happy camper. Apeks, Aqualung, Atomic, Dive-Rite, Scubapro and Zeagle (to name a few) all make regulators which are very popular among divers doing more advanced diving.
Oh, when he says cold water, we are talking in the 40's. I'd have a hard time imagining a piston freezing up here in Texas.
Personally I use Apeks DS4's on my backgas and Dive-Rites on my deco bottles (both diaphragm).