I had exactly the same questions going through my head for weeks. Finally i asked some guys on the french forum plongeur.com who owns various advanced monofins (Chen bin, ww one wing, leaderfin hyper....) and tested them in different hardness. As i owned a WW Model 1 Long distance (soft) and in their opinion the waterway and leaderfin soft fins behave very similarly, i decided to order a hyper in soft hardness with 70x70 dimension yesterday
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I also first thought to go for a harder monofin this time, but after training a bit more with my WW, i realized that even a soft blade could be used for sprints. It's all about technique. By undulating with a very high frequency i have no probelm with my blade at all, but if i push hard on the fin by using more my knees i will waste energy as the blade is not conceived for such loads. So in my opinion just go for the softest blade that is suited for
you. Now that's the tricky part, if you are a real heavy weight it may be a better option to get a harder fin. In the end the best would be to try out some monofins, even if they are not from leaderfins, and get an idea of which hardness suits you best.
My 2 cents
Christophe
