MikeFerrara once bubbled...
I've tried both. I really tried hard when I became frustrated trying to teach control using different kicks to divers wearing splits. the result of that effort was that I now rarely allow students to use split fins in class (never in a class like OW that deals with fundimental technique). After class they can use what they want but if they use them in class experience has shown that they'll not be able to perform, they'll become frustrated and they won't pass.
I hope this is a joke? So if a student wants a pair of split fins for his or her O/W class you will not allow them to train in the fins they will be using once they are out on their own? Who's job is it to oversea the proper fin technique regardless of fin style?