I would go with mediums regardless. I have always tried to mount everything as low as possible. I've always liked 36mm scopes just because I could mount them lower.
The Montana stock is designed right for use of a scope right off the bat,so low scope mounting isn't such of an issue as it is with stocks that came from an iron sight heritage.
The mediums will give you more bolt clearance and are actually more comfortable for me on the Montana even though I like a good cheek weld.
