Your choice is certainly a good one. But the MK25 is just the first stage. Scubapro sells their regs in a 1st/2nd stage combination. For example a very common MK25 reg is the MK25/S600. This is a very nice, high end, high performance regulator. The MK25 is a balanced piston 1st stage. Piston regulators are a bit more prone to freeze up in very cold water because the piston mechanism is more exposed directly to the water. If, as one poster suggested, you are concerned about regulator freeze up, then a better choice would be the Scubapro MK17/S600. Mk17 is a balanced diaphram regulator. Diaphram regs are more sealed away from direct contact between the water and the inner workings. So, diaphrams are less prone to freeze up.
I am a fan of Scubapro regs, but I suggest you stay away from the MK2/R195.
I am also a fan of Atomic Aquatics. The interesting thing about Atomic Aquatics regs is that they are all nearly identical design. The difference between the models is what material they are made of. AA uses Steel, Brass, Monel and Titanium in various combinations to make the various models. Also, the founders of AA came from Scubapro, and the AA 1st stage design shares many design elements with the Scubapro MK25. But with some of their own innovations.
I currently use both a Scubapro MK25/S600 and an Atomic Aquatics T1/Ti2. Both are piston regs. In the past I have used a Mares MR12II and a Poseidon Odin, both diaphragm regs. I dive in water that can get into the low 40 degrees F. The only reg I have ever had freeze up is the Poseidon Odin, a reg designed for cold water. Go figure.
(Epiloge: You might be thinking 40 degrees F isn't cold enough to freeze water, especially saltwater. But remember, when you release pressure from a compressed gas tank, the temperature drops a lot. It doesn't take long to turn 40 degree water into 28 degree water inside the regulator body.)
