You didn't say how you intended to use the gun. You did say something about shooting a seventh round at the range. Why would you be worried about that? In my mind, the only reason for a 7th or 8th round is to stay in a gunfight after you've fired your 6th round. At the range, this is irrelevant.
If you want to compete in revolver competitions, get the six-shooter because the other is not allowed. If you want extra margin when defending home and hearth, get the 686 Plus.
I went through this exercise a few years ago while deciding what to buy. I complicated matters by also throwing the eight rounder, the 627, in for consideration. I found the 627 too bulky for anything but night stand duty. Someone should check me on this but I believe the cylinder size is the same for the 686 and 686P but the 686P has less material between chambers. Thus, the 686 and 686 Plus should carry the same with the same barrel lengths.
If you buy the SSR, it only comes with a 4 inch barrel which is fine for general purpose. For home defense and concealed carry, I favor shorter barrels in the 2 to 3 inch range. This makes it much more difficult for an assailant to grab the barrel and wrench the gun away from you.
I decided not to buy any more internal lock revolvers and I ended up buying a nice used pre-lock, pre-MIM 686 with 2.5" barrel and I'm very happy with it.