I know divers have their personal preferences and brand/model loyalty but strictly from a functional stand point what are the advantages and disadvantages between balanced diaphragm 1st/2nd stages and balanced piston 1st/2nd stages?
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Aside from the environmental sealing stuff (you can environmentally seal a piston, btw), it's very easy to adjust the intermediate pressure on a diaphragm. Piston's will require shims.
Whether or not that's important to you, that's for you to decide.
also worth noting the diaphragm regs tend to offer better balancing in terms of IP swing. Even the MK25 has a 25+psi IP swing from a full tank to empty. Not important for balanced second stages, but for unbalanced regs it is noticeable
... it's very easy to adjust the intermediate pressure on a diaphragm. Piston's will require shims.
... From a service standpoint, pistons are a little easier to service but tend to require more specialized tools to do it properly. (bullet tools and piston oring installation tools) but once you have done a few, both are easy to service IMO. Service kits for diaphragm regs tend be be a little more expensive and many of these parts are not available other than from a dealer. Piston regs have few parts and with the exception of the HP seat, the orings are usually common sizes that can be purchased inexpensively at a number of places.
Both are reliable and work fine, which you choose is up to you . I prefer diaphragm regs but if I were stuck on a remote place where parts were hard to come by, a simple flow by piston reg (SP MK-2 for example) would likely be my choice. A bag of orings and I can make it work for many many years.
Any reg made in the last 30 years or so can flow WAY more gas than any 2 divers can breath so its a moot point.