(copy of my response on the same question in the instructor to instructor forum)
Take this list of regs to the poles (artic, antartic)...or Great Lakes and the order might just get shuffled around a bit. Ease of breathing vs reliability.
From what I have observed and heard from NOAA divers up here they have had lots of issues of Scuba Pros free flowing during spring buoy placements (32-34 degrees fresh water), Oceanics were a bit better, now I am seeing them using Poseidon's.
What I have read from Jeff Bozanic Antartic expirence posts:
Aga 1 (they have to be the most reliable, bailing out of an aga full face mask in freezing water might make lack of air a secondary issue)
Sherwood 2 (the next to most reliable reg but has QC issues and some are hard breathers)
Posiden 3 (almost the most reliable reg, a very easy breather)
But I have been diving with my Oceanic DX4's down to 32 degree's FRESH WATER temp and have only had second stage issues because of either getting them wet or pre breathing them before splashing. On the same dive I have seen USD Sea4's and Apek's come up with .5" of ice all around but not quite covering the entire reg and my Oceanic DX4's not have a bit of ice on them. None of them failed but one wonders if the dive had gone on another hour they might have.
This post asks "one reg for life", now you have to consider availability of parts for life. Gee this is hard. Sherwood doesn't redesign their 1st stage every year and parts would be common. Poseidon has few changes to their design as well but parts are difficult and expensive to get. Also Poseidon seems to change distributors every other year. Oceanic has been around forever and can be counted on to have a reliable parts inventory.
Should price of parts be considered?
Not if reliability in the enviroment you dive is in question.
As I dive doubles I think I would have two different type regs and wouldn't have a problem with Poseidon, Sherwood, or Oceanic. Sherwood would definately be the secondary air source. Dosen't always breath easy but a rock solid reg.