.45 can gets my vote. Running 147gr 9mm through my Osprey 45 isn't as quiet as it could be, I suppose, but it's still absolutely hearing safe. In all honesty, even the FRP sounds like a big nail gun (air powered framing nailer). After that, it gets a lot more quiet. I haven't shot it wet, so I imagine that that will be even better.
Back in April I bought a dedicated 9mm can (Octane), and after having second thoughts regarding the number of hosts on which it could be used, switched that order to another .45 can (also Octane). The extra length/weight of the .45 can is a non-issue for me, especially when I consider my use case for a suppressed pistol and the additional calibers that can be run through the larger can (in my case, .40, 10mm, .45).