Excuse me while I depart from the herd, but I like simple, reliable and low profile for my deco regs.
I currently use rather dated MK 3 and Mk 200 first stages with Balanced Adjustable second stages. A comparable "new" SP reg would be the Mk 2 Plus with a G250HP or S600 second stage. It does not come packaged that way but you can order it that way from your dealer. The "new" is in quotes as functionally the Mk 2 Plus is a Mk 3 with some detail changes and a slightly large piston head and the Balanced Adjustabel uses the sdmae basic design and the identiocal poppet as the G250HP and S600 and delivers identical performance - excpet in a more durable metal case with no potential for dry mouth.
I like this combination as it offers a first stage design that is:
1. Simple (only 1 moving part, only 2 dynamic o-rings, bith of whihc are in the intermediate pressure section of the regulator.)
2. Reliable (see #1)
3. Inexpensive (see # 1, plus they last forever and athere are a lot of used Mk 3's Mk 200's and Mk 2 Pluses in excellent condition out there that can be rebuilt to functionally new condition, or you can get a new Mk 2 Plus from Sp for comparatively little cash.)
4. Compact and low profile (they are small, light, and any of the 4 LP ports can be positioned so that the hose routes directly down the side of the deco bottle. With a DIN connection they are very clean and snag free - as good as or better than a Mk 17, Apeks, or Mk 25 second stage.
5. Unbalanced - contrary to the herd mentality and opinion, this is desireable as it results in a slight increase in inhalation effort once the tank pressure drops below 300-400 psi. This is in effect an excellent and very effective tactile cue that your deco bottle is about dry and you better be arranging for a gas switch. It basically eliminates the need for an SPG or perhaps a button SPG. In comparisona Mk 17 or Mk 25 will deliver no effort breathing right down to the last dreg of air in the tank with no warning and can leave you hanging after you have exhaled with no gas to inhale.
With an unbalanced first stage like the Mk 3 or Mk 2 Plus there is theoretically a slow and steady increase in inhalation effort constantly occurring as the tank presure falls from 3000 psi to 300 psi but in practice this is not usually noticed and in any event, if you have an adjustable second stage it is irrellevent as you can periodically and quickly tune any increase in inhaltion effort away.
As indicated above, I prefer an adjustable second stage, but if properly tuned, a balanced second stage or even unbalanced non adjustable second stage will work fine. A simple unbalanced second stage like the R295 has it's advantages as it uses a simple, reliable downstream poppet design and is also low profile.
So my picks in order of preference would be:
1. Mk 3, Mk 200 or Mk 2 Plus first stage with a Balanced Adjustable second stage (I love the small metal case and excellent performance, and you can still find good used ones out there and have them rebuilt by your dealer as parts support is still excellent and they use S600 poppets.)
2. Mk 2 Plus with an S600 second stage (great combination of simplicity, reliability and excellent no surprises performance.)
3. Mk 2 Plus with an R295 second stage (it has a little less performance but it is still very reliable and low profile and it costs a lot less money.)
4. Mk 17 S600 or G250HP (Small profile, reliable cold water fully sealed design that still allows the hose to run straight down the tank - but it costs a lot more.)
5. A Mk 25 S600 or G250 (over kill, expensive and the large L shaped first stage is more prone to snags, etc.)
