Rather than simply throw out suggestions, or respond to an 'either - or' selection, it might be easier if we understood why you were looking at these particular models (deyond a preference for Apeks / Aqua Lung). Either first stage would be fine for cold water diving. Why did you specify the 'military' DST? I have several DSTs and FSRs, and I use them in fairly cold water, and have never had any performance problems. Apeks first stages are very solid, and a bit heavier than some other brands / models, which is a good thing - it moves weight up toward your head, where most divers need it, particularly if you are diving thick exposure suits in colder water. I usually don't recommend turret regulators for single cylinder, backmount diving because I don't care for the hose routing a turret creates, although I dove one for many years. Of the two first stages I would lean slightly toward the FSR for single cylinder diving, but either one would be a good choice. And, if you ever decided to try a sidemount configuration, the DST would be a great asset.
For an octo choice, either unit would be fully functional. I happen to prefer the XTX50 over the 40, because it offers a User Adjustable Breathing knob, although that is not by any means an essential element. But, much of any poster's recommendation is going to be influenced by personal (subjective) preference, so you may also get testimonials from people who never use a second stage with UAB, and are perfectly happy.
