My guess is that you're on the cusp where either would do. I'm 6' myself and my bike is a large, but I reckon a medium would have been just as good - possibly better. Since I have legs that are a little short relative to my height I feel in retrospect I might have done better with a medium frame and then put a slightly longer stem on. However, I think you quickly get used to what you ride regularly so I wouldn't sweat it too much.