Hello divers!
I want to explain you my opinion about the topic piston or diaphragm first stage.
People who want to dive the whole year dont own regulator equipment for the different seasons, I think. Here in Germany, and northern Europe in general, at least the half of our dives are cold water dives with water temperatures below 10°C (50°F).
So witch regulator to choose? If the worst condition is the cold or dirty water, then you should choose a cold water reg, of course. Cold water regs work in warm water, too.
Looking at the construction principle I find piston first stages have one big disadvantage: the water has to enter the regulators spring chamber. So it comes closer to the cold spots in the first stage. This could make outer freezing easier, where water freezes around the piston and main spring (I dont know the exact English term for it). And when you dive in salty or dirty water, the matter of cleaning a piston first stage after the is also relevant, I think. Some recommended cold water approved diaphragm first stages today use a dry chamber construction to keep water away from the main spring and so also away from the cold spot (Apeks, Aqualung, Cressi , Scubapro MK17, Oceanic FDX10, HOG=Apeks, Zeagle). This is very simple and reliable and the main components of the first stage remain clean. Others have a totally different function principle (Poseidon XStream).
The solution with filling the spring chambers with a special liquid to prevent freezing can also be found, but I dont like this because it is extra effort what makes the reg more expensive and more difficult to service.
Because of the mentioned construction disadvantage, a German diving association, the VDST, does not recommend piston first stages.
Google the PDF falle_tauchausruestung on htsv.de and there especially look at the slides 9, 14 and 17. (some translations: Kolben = piston / Membran = diaphragm / Kälte = cold).
Of cause, any type of good regulator, which is approved to the European EN250 (standard for cold water diving equipment), independent from its type of construction, should work well in warm and cold water. In most cases even regulators without being approved for cold water work fine, if you have DRY AIR in your tank and have serviced the regulator properly. And many divers didnt have any problems with warm water regs in cold water. But will it still work in critical situations?
The matter of servicing effort/price should be no argument when we talk about safety. In Germany you will not pay much more for a service of diaph. regs in general. It depends more on the brand and their costs for spare parts. Cheaper/easier service may be a matter for diving schools with much equipment and very many dives a year, I think, but not for your own safety.
When you want to buy a regulator in Germany, nearly any dealer will offer you a diaphragm regulator here.
So I would always recommend diaphragm first stages, because I give more trust in them.
Btw.:
A regulator does not only consist of the piston or diaphragm. The construction in general is what really matters (because of that I trust Poseidon XStream most). And without good service every regulator can fail, even in warm water.
I would also recommend Apeks XTX 50/100/200, but I chose the Poseidon, because Poseidon requires service every two years only combined with a lifetime warranty and this gives me more trust in the technology when a manufacturer says so.
Both are fine. Pistons are a bit easier to service, maybe, just maybe perform a wee bit better. Diaphragms are said to be better for cold water and are a a little bit more complicated to service. That said, I dive my piston scubapro down to temps of 45 degrees w/o issues - same with my old school Sherwoods.
