Split Fins
When I use split fins, I use a set of Atomics. I've tried several different kinds, including those by Sherwood, Scubapro, Biofin, etc., and usually condsidered them to be too soft feeling. I found that the atomics were a little bit stiffer on the outside rails which seemed to give better control when sculling or using a frog kick.
You'll note I said when I use splits as I have become more conscious of the benefits of a non split fin in certain circumstances. The split fins seem to give me good propulsion in calm conditions when I am diving with minimal gear (i.e., tropical diving). When I started dry suit work, doubles and/or spearing I found that the splits were not nearly as versatile. With a split fin I used less air to move around on the reef, but the actual amount of power I could derive from a fin kick seemed to be less. I may have to work harder to move with my old Gorillas, but by gosh I could fight a current and perform finer movements. Check out the thread on the Hunting Equiment forum "Fin Lineup" started by Unirdna.
I'd also like to plug the fin by Aeris, which tries to provide the best of both worlds (is this like trying to have it both ways?). This is a non-split fin with a more pliable slotted material spannig the center of the fin. Its supposed to perform like a split fin for propulsion and also provide some of the benefits of a non-split. I don't know if they do all that, but I've dove with them and the seem to be a pretty good little number to dive with and are less than $100 US vs. $175 for the Atomics.