Terrific thread, Sam! I'd be happy if more people post their standpoins here!
I'll quote a friend of mine, experienced diver, who always says that the fins are the most important and in the same time the most individual part of our equipment. Especially that concerns the spearfishing.
Let's see how different are the requirements for the fins if you hunt near the shore, in shallow waters and rocky bottom or, in deep spots where you dive mainly in vertical. Will you reach the spot by boat or will have to swum a lot? What hunting technique do you apply - static ambush or dynamic search, how deep and what amount of ballast weight you need in the particular case...? That's from one hand. From other - what is your physical type - height, weight, muscularity, in particular - leg's length. Third important thing is the combination of your personal experience, skills, style of swimming and temperament, etc...
Here I see and the main reason for the countless variety of blades and specifications on the market - for everyone to his taste...
As you can see, it's not easy a certain advise to be given. There isn't any guarantee if someone is satisfied of his fins it'll be fine for you too...
That's why and many spearos have two at least, often and more pairs of fins for different diving and hunting conditions...
If you can not afford it then buy some fins with average performance - middle to soft, not so long (76-78 cm), with soft and comfortable foot pocket and flexible side rails and go diving - the practice will show you how your next fins will look like...