@mainedvr if you have hog D3's, the parts kits are interchangeable.
Here is what I have and why
Primary *doubles/CCR or sidemount-Scubapro MK25, Poseidon Jetstream
Bailout/Stage-Poseidon 3960, Poseidon Jetstream
Deco-Poseidon 3960, Scubapro 109
MK25/Jetstream because
@victorzamora gave me sh!t for years about not having swivel turrets so I caved and got a good deal on some. Admittedly they are more comfortable in sidemount, and obviously offer a lot better hose routing in backmount.
I love Jetstream second stages, so I keep using them. I am not a huge fan of pistons in general vs. sealed diaphragms, but as I don't actually own any sealed diaphragms at the moment for a myriad of reasons it may sound a bit hypocritical. My diving is primarily freshwater caves though.
Bailout/stages I use 3960's because they're tiny and are really nifty little first stages. Same reasoning as above for second stages.
Deco is same reason as bailout but use Scubapro 109's because I can put an inline shutoff on them easier than the Jetstreams and they give me one extra layer of caution before I put that reg in my mouth.
If I was diving in the ocean all the time, I'd use sealed diaphragms because keeping the salt out is a lot easier. For example, the stage bottles, which have unsealed first stages are a right PITA to get sand out of the chamber and I have seen some UGLY top sections when buying those used that have been around the salt. Same with unsealed pistons. If you don't have your own tanks, and aren't going to soak them completely before they dry, preferably with water moving *I have a fish tank circulation pump that I stick to the side of the bathtub to keep it moving if I'm diving in the salty stuff*, then I'd really go sealed diaphragm.