I dive with piston regs and feel that they offer advantages in terms of both flow rate and performance and the freeze problems some divers report has been a non issue with me.
I dive in water in the low thirties in winter and in the mid to upper forties at depth in summer and have not had a problem with the Scubapro MK 3, MK 10, MK 15 or more recently the MK 20 or Mk 25 first stages.
The TIS kit (Thermal Insulating System) used by Scubapro is not, in my opinion, as effective as the SPEC (Silicone Protected Environmental Chamber) kits used on their older MK, 3, MK5, MK 9, MK 10, or MK 15, but it seems to work for me.
The piston reg is also bullet proof with essentially one moving part (the piston) and very few critical o-rings subject to wear (two, 1 high pressure and 1 low pressure, on most designs) from the moving piston. They are easy to maintain, require minimal service and are easy work on for annual service.
In terms of mechanical relaibility, they have a distinct advantage over more complex diaphragm first stages and will maintain peak performance bewteen regularly scheduled servicing with no potential for degradation caused by stiffening rubber diaphragms, etc.
Some divers do have problems with the newer MK 20 and 25 in cold water, but other don't and I think it relates to breathing rates or patterns. With the older MK 3 or MK 10, I can fill a 200 lb lift bag at 140' in 40 degree water and not freeze one up.
But if you are diving warm water, I would choose a good piston reg hands down over a diaphragm reg just for the increased reliability and performance.
The MK2 is basically a redesigned MK 3 that is DIN compatible and has an extra LP port. It is not really a great performer at depth but is simple and reliable.
The R190 is not balanced and if not adjusted properly can freeflow (a not uncommon complaint) particularly with an unbalanced first stage like the MK 2. If you get one, it needs to be adjusted for minimum breathing effort on a full tank. The intermediate pressure varies a bit from full to empty tank on an unbalanced reg as the high pressure air from the tank acts directly on the seat mounted on the bottom of the piston.
What often happens is the reg gets adjusted on a shop air tank at far less than 3300 psi. the result is that when attached to a full tank, the intermediate pressure is slightly higher than when the second stage was adjusted and the reg then has a slight freeflow. (Some techs are really poor at their job and the reg unfortunately gets blamed) With proper adjustment freeflow is not an issue but, being unbalanced, it will breath slightly harder at low tank pressures.
The R380 is a balanced second stage and offers consistent performance with varying intermediate pressure, but I would go with a G250. It's balanced, adjustable, bullet proof, and well matched to the Mk 25 first stage. Although it is not marketed this way by Scubapro, it also works well with the Mk 2 first stage and if set up properly the G250's adjustment allows you to maintain minimum breathing effort as tank pressure falls.
With a Mk2 and either a G250 or an R380, you would have great performance at moderate depths for relatively small bucks. With a MK25 G250, you would have a reg that would go anywhere and give great performance.
Scubapro has also been very reliable in terms of staying in business, providing accessible service (even if you do have to tell the tech how you want your reg set up now and then), and providing long term parts support. I can still get parts for my first reg, a 20 year old Mk 3 high performance, not many companies offer that kind of long term parts support.
