On my doubles I use a Beuchat VX10 on the left post, and a Scubapro Mk20 on the right post. I like the combination, and how I am able to route the hoses. By turning them sideways, everything goes where I want it, and they do not interfere with vavle operation. Both are excellent breathing regs IMO. I have never used them in cold water though, so I can not give an honest answer for that department.
My dive buddy just got a Apex ATX200 that he won from the Apex website. The thing is super sweet. I will have two of these in the future for my doubles rigs. The hose routing options with these things is unbelieveable. Plus the fact that they are so tiny! I want these!