MK 25 or MK 17 for Cave Diving

[Sludge]

why do you prefer the piston over the diaphragm for water above 40?

If I may be allowed to plagiarize a bit, "first stages with air balanced pistons deliver significantly more air to the second stage than any other first stage, while their performance is totally unaffected by the changing tank pressure. A balanced piston allows the use of lighter and more sensitive components, resulting in ultra-fast breathing response, instant delivery of air on demand, and extra high air flow, especially in low tank pressure ranges."

When I started diving, the store owner was so adamant about this that he wouldn't even sell diaphragm first stages. Believe it or not, this is more important in single-tank diving than diving doubles, because there is almost no scenario where two divers will be breathing off the same first stage whereas an air-sharing episode of single-tank divers will involve two divers and one first stage. Doing this at 100' will be beyond the air-delivering capabilities of a diaphragm.

Also, I have a potential reply to your theory of not getting service every year, but I'm not sure I want to go out on that limb.