I actually just bought a new pair of fins, and I'm eager to try them out. I read a lot of reviews in dive magazines and diving websites to see what each fin was known for (power, ease of kicking, etc.) and what reviewers had to say about them. I also talked to several divers I knew who had split fins, paddle fins, etc. to see what they liked about theirs and if they would recommend them. I can tell you, the divers I talked to were firmly in either the split fin camp or the paddle fin camp; I think I spoke to only one person who liked both types of fins!
I was also influenced by what I *didn't* care for in my prior set of fins. For example, I got caught in a strong current once and had a difficult time getting out of it because my original fins didn't have a lot of power built into their design. So "power when I need it" was one of the things I looked for.
(P.S., it also didn't hurt that I got a small discount on my fins through my local dive shop because they had been used only once in a pool and returned. I didn't mind the one-time use as they were still practically new. )