I think you posted in the wrong section, as a monofin would be classified as one 'fin' not two 'fins' :t
I'll help you out anyway, I can't remember the exact reasoning behind closed heel but I believe it's a comfort and power transfer thing, freediving fins are all closed heel as far as I know, so there is not too much choice in that point.
The hardest part is to find pockets that fit your feet well, much like a mask needs to fit. Personally I like Beuchat pockets as they are a bit narrower, quite soft and very comfy! Picasso pockets on the other hand are wide, and very hard, they hurt my feet, ankles and knees.
I was always a believer that stiffer the blade the better, but I realize now after some research that this not necessarily true, it's more about getting the correct curve for maximum efficiency. If you are a strong guy then stiffer fins would probably suit you, but I would still prefer mediums over hard.
Carbon fins are the bomb. Fiberglass are pretty good too, but seriously carbons are amazing. Try and find a pair to try out; but I warn you, once you try carbon fins you can never go back to plastic!
I personally use DiveR carbon fiber blades (Med to Hard blades) in Beuchat pockets. They are probably a bit too stiff but I'm pretty strong for a skinny guy and they really get you moving!
-James
