It's not the mount that makes a difference. It's the fact that you can only run a certain size pinion on the outer. Then you switch to the inner for larger pinions. I bought the inner since I run 21t pinions.

And Jason, I just bought 1 for now. Had a few $$ to burn and I was bored. I'm trying all different kinds. Trying to stay away from the Reedy #720. But I'll eventually get one.