I own both- the longbladed Free Dive Fins and standard Force Fins.
There is a purpose for each. If you have the legs, and if you have the need, the long blades can deliver. For everything short of dragging a man 300 yards, I'd go with my Force Fins.
I have never seen a pair of Long Blades that retailed for even 1/3 of what Force Fins are MSRP'd at. Long blades go for $80~140. Just saw a wall of them at Divers Discount in Ft. Lauderdale.
I do not believe that these two types are on the opposite ends of any spectrum, it's just that most people believe that Force Fins do occupy the low thrust zone. They do not. They are equal to most any other fin and will allow you to deliver that thrust with less exertion.
When I am going in open water, I usually take the free dive fins. When I am anywhere near coral, I don't want to risk any damage, and I have no need to go that fast, even on short bursts.
If you select free dive fins for SCUBA, you should have the legs of a good cyclist first. And- stay away from the coral.