Large would be fine, most bindings come in L/XL anyways. Boas break all the time, especially if you like your boots super tight. They always seem to break at the worst times. Not to mention you don't have nearly as much control as to where the boot actually tightens. The only real benefit is that they are slightly quicker to tie...but lets be honest here, are you really that lazy that you cant tie up your boots once a day?
I'm curious as to what good things you heard about them?
How many days a season are you riding?