To me it comes down to kicking style. IMO, someone with a bigger flutter kick is going to easily overkick a split fin. As a competitive swimmer for many, many years, when I tried a split fin it only worked well for me when I did small flutter kicks. I felt that the propulsion was excellent as was the speed and "quickness off the line". However, when I did my normal flutter kick, I easily overwhelmed the split design, and they were basically ribbons on my feet when I did a power kick. Because I'll naturally go to a bigger kick when I need more propulsion, the splits are counter-intuitive as a fin for me. When you add into the equation that I prefer to frog kick anyway, the splits completely fell apart as any sort of option for me.
Like everything else, it's all about trying as much gear as you can BEFORE you buy and finding what is best for YOU. And I highly recommend that nobody listen to the recommendation of anyone without trying for themselves IN THE WATER.