For deco regs, I prefer a simple unbalanced piston first stage.
An unbalanced piston frist stage is, in my opinion, nearly perfect for 50%-100% O2 use. There are no high pressure areas in the first stage past the high pressure seat and both dynamic (as well as nearly all the statc o-rings) are exposed to no more than intermediate pressure (140-145 psi plus ambient water pressure). They are also first simple with very low parts counts and onlt one moving part.
Scubapro does not market the Mk 2 Plus for 100% O2 use, but it does use EDPM o-rings and Christolube so with proper O2 cleaning, it is suitable for high O2 applications.
I like to combine this with an adjustable second stage like the the G250 or S600. It allows very low inhalation effort during deco but also gives you ample warning that the deco tank is about empty as the inhalation effort begins to rise noticeably as the pressure falls to the 300-500 psi range. In contrast with a very high performance balanced first stage, it is not uncommon to take a normal feelign breathe, exhale, and discover the tank is empty - which leaves you holding your breath on now empty lungs while you make a gas switch.
Thsi feature also means you can dive effectively with no SPG on the deco bottle, although I still use a button SPG on the first stage to confirm the tank is full prior to the dive and in a pinch you can in fact still see them underwater.
My favorite deco reg is a Scubapro Mk 3 or Mk 200 (basically the same as the current Mk 2 Plus, but with one less LP port) with a an all metal balanced adjustable second stage. The metal second stage eliminates dry mouth and the design uses a full size diaphragm G250 performance but in a smaller package than a G250 due to the thinner and stronger chrome plated brass case.
