In an attempt to be more clear, I'll try again. Lets pick a balanced piston (unsealed) at one extreme and a balanced sealed diaphragm at the other. Again we're talking about first stage only and using IP stability as the measure of when a rebuild is warranted.
Which, in your experience, requires a shorter service interval. (I had thought the MK17 was a balanced regulator, thanks for the education)
The MK17 is a balanced regulator, meaning it does have a balance chamber. But it is not fully balanced; there still is significant upstream bias, so the IP rises as the tank pressure decreases. With a high performance balanced 2nd stage most people don't notice this as the 2nd stage compensates for changes in IP to a degree.
But this is not the same thing as "IP stability". Unstable IP due to wear on the seat is always experienced as IP creep, meaning the reg does not lock up quickly and stay put after each breath. The rise in IP over the supply range in the MK17 is by design, it will do it when its brand new or 5 years after a rebuild.
If you are going to compare piston to diaphragm regulators, you have to consider that flow through (balanced) and flow by (unbalanced) piston regulators are very different in design and function. Some flow by regulators are famous for tolerating abuse and lack of maintenance for years.
Pistons are stainless steel and as such are very unlikely to rust, but there is some validity to the idea that bits of crud can contaminate the HP o-ring on the ambient chamber side in some flow through piston regs. Most of the newer ones (like the MK25) have bushings that tend to protect the HP o-ring, and I've seen MK25s go for many years before creeping. The classic USD unsealed diaphragm design has likewise proven itself to be VERY stable and tolerant of lack of maintenance.
In all unsealed regulators, piston and diaphragm, there's an ambient chamber where water goes, and water does not penetrate any further. In piston regs, a portion of the piston shaft and head, the spring, and 2 o-rings are in this chamber and exposed to water. In diaphragm, only the spring and diaphragm (and maybe a spring pad) are exposed. So one might think that diaphragm regs would need less maintenance as a rule, but there are lots of other factors. Exposed threads are present in both designs, and diaphragm regs have that tiny HP o-ring in the balance chamber.
I have MK5s that are going on five years without any sign of creep, I've seen MK25s go for similar lengths, and I don't think I've ever rebuilt my MK2; I don't use it much, but I've had it for at least 10 years. So piston regs can certainly hold up.
If I were offering advice about getting regulators that can go the longest without maintenance, I'd say it's more important to get one of the classic designs that has has proven itself over decades of use than it is to choose a diaphragm over piston or vice-versa.
