Unless you're sure because of previous experience, that you have a particular need for a softer or harder fin, a medium stiffness carbon or fiberglass fin is always the way to go. Achieving a medium stiffness fin does not only depend on the blade, it depends on the footpocket/blade combination, as some footpockets have stiffer tendons than others which affects the overall stiffness and flex of the fin.
Because of this the first step in the process of getting the right fin for you is determining the footpockets. If the footpocket doesn't fit well you can have the greatest carbon blades, but it will mean nothing if the footpocket doesn't fit right and/or is not combined with the right stiffness/flex blade. Once the right footpocket is chosen, the blade is matched to it.
Disregard the numbering on the footpockets as a guide to what size is right for you, use the numbers as points of reference only. If you don't have the possibility to try on different footpockets, your next best move is to talk to someone who knows how footpockets from different manufacturers fit. In this case you’ll need to provide your actual foot measurement. Put a ruler against the wall, and stand on it with your heel against the wall. Putting your weight on the foot ensures that it's spread out to maximum length. Measure where the furthest toe ends. Measure the width of your foot at the widest part in the same way. This is not guaranteed to get you the right footpocket but it will narrow things down to two or three choices.