Leupold VX-R 4-12x?, do I get the 40 or 50mm

[gmantwo]

Ok, so I built an awesome new AR, or actually my friend did most of the "building", I just chose and gathered parts. I put a Leupold Mark AR 6-18 scope on it and am not real satisfied. The scope is too low to be comfortable(I used the Leupold Mark 2 integral mount, I'm going to add a Leupold rail to get it higher). And I think I want to try the VX-R scope to see if I like it any better than the Mark AR. But I'm undecided whether to get the 4-12x40 or 4-12x50. Not planning to shoot this in 3-gun, just targets and the occasional critter. I have NO experience with 50mm bell scopes, never owned one. Are the advantages significant? or even noticeable?