I use SP Mk25/g250hp for primary and deco/stage and SP Mk25/R190 for my backup reg.
Rational: Proven regulators, great hose routing for both backgas and as a deco/stage. If I was having a really bad day, I could swap a stage reg onto a deco bottle due to the piston design. The g250hp's can be adjusted to reduce free flow possibility (useful when then reg is parked). The R190 breathes just fine when paired with the mk25, and its even better with He in the mix. It has few working parts, so reliability is hypothetically increased, which is good for a backup. The face plate can be removed if needed to clear crap out of the reg. Also, most (if not all) of my buddies use the same regs (or permutations of them, like the s600 or the Mk20). Scubapro dealers are world wide, making repair easy.
Caveat: I don't dive in cold water, and I cannot comment on their performance in freezing conditions. All I know is hearsay with no direct experience to back it up.
Out of curiosity, if you have Mk25/g250hp on your right post and your deco bottles, and the R190 on your left post, why not put the R190 on the deco bottle and have a matched set for your doubles?
Well, a few reasons.
My backup reg NEEDS to work. R190 is a simple reg, and simple = reliable in this application.
On deeper dives, I spend as much time on my deco gas as I do on my backgas (usually more). A nice breathing reg makes deco more comfy, and the g250 has the ability to be tightened down so it doesn't free flow and drop the gas out of the reg if I leave it somewhere in the cave.
That is pretty much the stock DIR answer - an easy breathing balanced primary second stage and a "more reliable" unbalanced backup second stage. The reasoning is however a little flawed.
If you compare the R190 to, for example, a G250 (the original) or G250V, you'll find that the balanced G250 or G250V is not much more complex and in osme ways is less complex. The R190 uses a simple valve where spring pressure offsets 100% of the downstream force of the gas coming from the first stage. This higher spring pressure mans more pressure on the low pressure seat which accellerates the wear on the low pressure seat. This is significant as the most likely failure is a leaking LP seat.
In contrast the G250/G250V is a balanced design where much of the pressure needed to seal the valve comes from air pressure, not spring pressure so when the reg is depressurized (99.9% of the time) there is less pressure and less wear occurring on the seat. In terms of complexity, there are two o-rings that seal the poppet in the balance chamber, but one of these is a 100% redundant backup so the system in reality is no more failure prone than the R190 poppet, and is less likely to develop a lP seat leak and if it did, the adjustment would let you resolve it during the dive in most cases.
So personally, since it not only needs to work, but also needs to be as capable as my primary reg at depth, I'd go with a balanced second stage on both my primary and my backup.
I agree with you on the need for a decent second stage on the deco gas given the amount of time spent on it. But I also prefer the bullet proof simplicity and very O2 freindly traits of an unbalanced flow by piston design like the Mk 2. It offers more than adequate performance at 130 feet or less and when paired with a balanced and adjustable second stage like the G250 offers very low inhalation effort with adjustability to accommodate the drop in IP from the unbalanced first stage. And when the tank pressure gets below about 300 psi, the inhalation resistance goes up warning you the tank is about to go dry.
On a stage bottle that will potentially be used at depth, I want a high performance balanced first stage just like my back gas. The Mk 25 offers more gas delivery at depth, but the Mk 17 puts out more than enough gas and is also fully sealed making it much more suitable as it is bullet prooof in water temps below 45 degrees where the Mk 20/Mk 25 has been known to freeze open.
I prefer the hose routing on the Mk 17, as it routes all the hoses straight down with less clutter than the odd angled routing using the end ports on the Mk 20/25.
So my picks are...
Back gas: Mk 17 with a G250, G250V S600 or X650 (with the Mk 17 G250 being preferred).
Deco gas: Mk 2, Mk 3 or Mk 200 with a G250 or G250V (20 and 70 ft bottles) Mk 5, Mk 10, Mk 15 or Mk 20/25 with a G250 (120 bottle) I prefer a piston reg here due to the improved ability to swap regs in water between deco bottles if the need arises. Freezing is not usually an issue, and the valve can be manually operated if this occurs anyway.
Stage bottle: MK 17 or Mk 25 with a G250
