I think you would be fine with either of them. My girlfriend got them in Small and it fits her 6.5 boot. She does have a slightly bulky boot so that may or may not make a difference for you.
I would suggest going Small just because I would rather it be a pain in the ass to ratchet a strap all the way down to the smalles size then having a problem barely strapping in every time.
I see what you're saying. Thanks for the advice!