The MK25 is a piston design.
The whole middle part of piston regs is open to the water (inside there are the 2 main dynamic o rings).
For someone that was sold on the enviro seal of double diaphragm regs this is a step backwards.
This is a little misleading. Both piston and diaphragm regs have an ambient chamber where water flows to allow for depth compensation. On diaphragm regs, the parts that are exposed are the outside of the diaphragm, the main spring, the spring pad, and usually some sort of diaphragm retainer; sometimes this is just the endcap of the reg. On piston regs, the exposed parts are part of the piston shaft and head, the mainspring, and part of the reg body. There are two o-rings, one on the piston head and one on the shaft, that seal the ambient chamber from the pressurized internal chambers. In very cold water, it's easier for the piston shaft to freeze because rapidly expanding air cools the piston to well below freezing, and in water that is cold enough, ice can form on the piston shaft. That's the primary reason that diaphragm regs are generally preferred in very cold conditions. The rapidly expanding air is not in nearly as close proximity to ambient water, and so freezes in the 1st stage are less likely.
But to say that piston regs are not as durable or reliable because of this is not true. There are plenty of MK5s around with 30 years of service that still look and work fine. All 1st stages need to be soaked in fresh water after use in salt water; even sealed regs will take a beating otherwise. It's just a little easier to rinse off all the saltwater in diaphragm regs, at least in the older, simple USD (now aqualung titan) design.
Don't make a decision based solely on piston vs diaphragm. The most important thing for the OP, buying a new 1st regulator, is to get yoke if you're diving yoke tanks and DIN if you're diving DIN. Lots of SB regulars are pushing HOG regs; at least one is selling them, and they do seem to be very high quality for the price; I've never dove with one, so I really don't know. But, it's DEFINITELY a mistake to buy a DIN reg if you're diving yoke tanks, and at the moment, HOG comes only in the DIN configuration.
Aside from that I'd stick with a major brand and try to avoid buying anything with a lot of gimmicky features.