I wear a 10.5 Nike ZF1 with union forces. I bought the XL. There is plenty of room to size them down. The m/l would also fit because I'm sure they room to go up. It's funny because the 10.5 is right in the middle making the size a tough choice. Do you plan on staying with the Burton boots for the life of that binding? If you were to go to a non-shrink tech boot would it fit the medium? That's what led me to the L/XL.
I buy from my local shops but get plenty of hook ups from friends. I don't feel bad about "screwing over" my local shop because they don't mind selling me stuff that's marked up 40-60% or more. Just sayin.