The idea behind split fins is that instead of just propelling the diver forward, the split creates a vortex that assists in propulsion and speed. Another benefit of splits is that the opening allows water to pass through easily on the upward fin stroke. Since the upward fin stroke is the movement that achieves the least amount of propulsion, having the split is more energy efficient than having to kick upward with a standard blade fins. This makes the split fins easier to use in many cases and allows divers to conserve air because they don’t have to kick as hard.
As a result, split fins are good for casual divers using a scissor kick, inexperienced divers who may not have good kicking technique yet, and divers who have ankle or knee problems and cramp easily. However, blade fins are more powerful, and many divers argue that split fins do not provide enough power to fight through tough currents.
So, split fins are more efficient in terms of propulsion and oxygen conservation but blade fins provide more powerful thrusts. The type of fin used should depend on the type of diving.