For me, I would keep it on the trailer. But I suspect that I have different constraints from yourself.
You say that the time (cleaning vs launching) approximately balances out, so that is not a deciding factor. The only issue here may be - can you always afford the time to launch before you go out each time; sometimes it is easier to get all the 'pain', in this case cleaning, over in one big hit each month.
Given that all the factors, except $, seem to cancel out, the factor for me would be that a boat on the water inevitably suffers greater degradation than if dry berthed, no matter how good the cleaning. Certainly this was the case with our club dive boat. So dry berthing/trailer gets my vote.