I've got a DP Pro on an AR15 pistol. Works well. Can't say about the rain. Can say that rain messes with a scope, too.
I do have BUIS on my AR15 pistol, so if the DP Pro isn't usable I'll use those (just like I would on an AR15 carbine/rifle if the optic wasn't usable).
There have been reported issues with battery connection loss in RMRs. Some people just put a piece of electrical tape on the inside of the cover to insure the battery stays in connection with the terminals.
Is field of view an issue with a "1" X optic? I don't pay attention to the frame/lens on my RMRs or the DP Pros, I just make sure the dot is on the target.
When you shot iron sights do you really pay attention to the size/shape/appearance of the rear sight? When you shoot with a std. rifle scope do you pay attention to the "ring" around the sight picture or just put the cross hairs/dot in the right place on the target?
As you said, if you don't like it on the AR, you can put it on the pistol.
Right now I've got two RMRs (dual illuminated models = no battery) and two DP Pros. Can't say one is better than the other, but I can say I keep fighting the urge to buy one or two more, of either brand, so I can install them on another pistol or two.