I'm really not sure either, as everyone is different. The following is another excerpt from the Deep 6 website regarding sizing of these fins:
Sizing
We have found that the large works well in wetsuit boots with a flexible sole in sizes 9-11, the XL 11-13. When wearing wetsuit booties with a thick "sneaker" type sole if in doubt get the next higher size. Remember that a normal shoe is sized for the outside dimensions of your foot, which stays pretty consistent, Fins however we have to size to fit the outside of whatever boot you are wearing with them, and they vary greatly depending what type of boot you own. Don't hesitate to call with any questions as you choose a fin size.
They also offer a Small/Medium size. You seem to be right on the border between the S/M and Large. Perhaps check around this site, there may be someone with a review of that size that can inform you of how it fits/what size boot and shoe they wear.