Your sight rings moght answer this for you. I had to go with 40mm because the 50's would hit the barrel with the scope rings that came with the gun. (Ruger . . . love it!)
I can't see any reason to go with the 50mm unless it is a high powered variable say up to 14 or more. The higher you go in power with any optics, the more light you loose so the larger objective will let in lore light. Look at the objective on most spotting scopes.
As far as which scope? I went to buy a Leupold (only scope I'd used for 30+ years) and came home with a Nikon Monarch on it. Much better light gathering, clearer view, easier object chase, and less money. Worth a look.