I have a Remington M700 BDL Mountain rifle in .308 win, I had a simmons 44 mag 3-10x44mm on it for years worked great, but recently switched to a Mueller 3-9x40mm IGR sport dot. I like the 40 mm a bit better since i was able to mount it lower to the gun using low rings instead of the medium rings I had with the 44mm lens. I would say in your case choosing between the 36mm and the 40mm the only thing different would be the 40mm would have better light transmission. I personally would go with the 40mm unless your mountain rifle will be used for what it was designed for, then go with the 36mm. If you are gonna use it as a multi purpose hunting rifle then the 40mm would be great but a mountain rifle was designed to be compact and light to better suit a mountain hunter.

go to this link this will explain what I am trying to better:
http://www.chuckhawks.com/mountain_rifles.htm