When my mares first got sized they were sized wrong, but we didn't do the sizing, our farrier did (we had several issues with him, a major one being that he left a gate open once allowing the horses to get loose...needless to say we don't employ him anymore). This is when I was having the issues of them falling off. When we did have my girls re sized we found that one mare in particular was wearing a whole size too big! No wonder they were falling off when cantering! Lol!
I this case, because it's such a small difference between one size and the other, and with my personal experience with over-sized boots, I'd go for the smaller size. Just in case.