I read on the Internet that you'd die if you use split fins.
Which is the absolute truth.. it is also true for paddle fins... or just being alive.
I have several pair of each type and a pair of flip fins... my take is:
1. Fastest - the really soft split fins... they don't "seem" like they are doing that much, but have a stop watch and do laps and nothing is close.
2. Best in current - see above (but don't expect to "feel" like they are doing that).
3. Best for a frog kick - paddle, if you are just going slow. Splits can do a very fast frog, but it takes a very fast stroke.
4. Efficiency (lower air usage for the same distance and speed )- No question it is the soft split fins.. second place is not close.
5. Middle path (following the teachings of Budha) - stiffer split fins... not as efficient, but do an ok frog or any other kick.
As the soft split fins work best with a single type of kick (very different from the stiff paddle fin kick)... and they use the big leg muscles.. they are the choice for efficiency and covering a large distance.
But, if you dive every day, or for very long times, and want to used different kicks to rest parts of your legs.. then paddle is the way to go.
If you ride a bicycle.. splits are great (similar motion)... if you have strong ankles.. paddles are fine.
Hummm.. guess that is why I own both.