I work full time as a service tech and have been diving for 19 years, sidemount diving for the last 8 and CCR for the last 5.
I just finished my full cave in Mexico in March and found my and looking to get my last set of sidemount regs that are quite nice. My shop sells all major brands and is an authorized service center for many of the smaller ones, my only opinions on the matter of equipment is what works best for the application and what I've seen as common issues with equipment.
First and foremost when weighing my options for what to buy I decided any flow through piston was out of the running. Sure they breath great but the knife edge of the piston is highly susceptible to scratching and damage. If this knife edge starts to get wear you're going to see IP creep happen. IP creep on a reg that will sit unused underwater for any length of time is likely to eventually lead to a small leak from the second stage. This takes MK25's & Atomics out of the running.
This is kind of sad for me as I dive the ST1 for my recreational single tank backmount dives and absolutely love it. I also love that Atomic makes it in stainless steel as this has been an amazing reg for me and if you take even decent care of it, it will probably last a lifetime.
That means that only diaphragm regulators are options for me.
Of diaphragm regulators I decided they had to be environmentally sealed to help protect them from corrosion and turbidity in the water. They also had to have a turret on the first stage.
That leaves only a few options from the major manufactures and then the same design from the smaller manufacturers that do it at a fraction of the cost.
Aqualung is out because they don't have a sealed diaphragm design with a turret on the first stage (that's a shame because I love the ACD design)
Hollis DCX would work but I don't live the seat / knife edge design and I've seen an issue with a few of their seat / knife edges from the factory supplied service kits.
We're left with Apeks XTX50's & the smaller companies knockoff's of it.
I own DiveRite XT1's and they do a great job but recently after servicing mine I had a major issue with the body of the first stage splitting in half. Yes, the whole body physically was broken in 2 which was the first I had ever seen that on a reg (I have a picture of people are interested). Ultimately the reason a lot of these smaller / newer manufacturers can offer such lower prices is they sell directly to the end user and they source materials from lower quality.
Apeks has been doing this for a long time and has quality metal so that was what I went with for 1st stages, XTX50's. Also if you keep up with service / inspection records you can get service parts for life with these which at retail cost saves you about $100 every 1 - 2 years.
As for second stages any balanced second stage is going to give you pretty great performance with the biggest difference being the amount of spring tension you can adjust, some small manufacturer specific features and the material used.
I ended up going with the atomic ST1's for second stages (no swivel hose) for one specific feature, the seat saving washer. One of the parts of a reg that can cause them to need service will before they've gotten good use is the wear the 2nd stage orifice and seat get from just sitting. The seat saving washer will prolong the life of the seat and prevent you from needing to get the second stage serviced if it just has a groove worn in it from sitting unused for a few months.
Ultimately I wish the atomic 2nd stages had a bit more of a range on ability to adjust the spring tension but this is minor compared to having a small leak due to seat wear.
The only better combination I could think of would be an XTX50 clone made out of stainless steel.
