You are really asking about sealed diaphragm vs. piston in this
Deep doesn't matter
Dirty, the dry sealed diaphragms are much better than the sealing used for pistons in nasty stuff
Cold, the dry sealing isn't necessarily better, but it is much cheaper in the long run since you have about a $50 usd upcharge to seal pistons during service.
The following questions aren't pertinent to piston vs. diaphragm, but are highly relevant to which regulators to choose once you narrow that down.
Location: parts and service support are different for certain brands
DIY Service: huge consideration when choosing a new regulator
Under those criteria, I would personally choose either a Poseidon Cyklon or Xstream, but I don't recommend them to people unless they have tried them first. I would choose sealed diaphragm every day over a piston for those circumstances though. Deep6 will be top of my list starting next week when Chris releases them officially, followed by Dive Rite XT, then Hog, then Apeks. This is due to cost/quality and Apeks is at the bottom due to lack of parts availability.
edit: please ask the mods to merge this with the other thread you created as I think it accidentally double posted