I'm sure not every spot has the luxury we do down here in so cal.. so many of our scuba shops have pools right inside or next to the shop. And the majority of them would love for you to try on a brand new (or special trial versions) in the water BEFORE you buy.
I recommend that you try:
1. A split fin
2. A classic fin
3. A stiff fin
If you have already been instructed in proper finning techniques do the following, otherwise get a quick reminder from a professional, if you are swimming incorrectly during your demo the fin you swam with improperly MAY BE THE WRONG FIN when you get around to doing it right. Anway try a few different maneuvers:
1. Swim in a gentle circle right then left (pay attention to what you are straining or what special motion you are having to make to accomodate the fins.)
2. Perform a short duration series of power kicks in a straight line, sort of a get-out-of-danger scenario.
3. Turn around in place right then left. How well do you stay in one spot, making these very tight in-place turns?
By the way I recommend that you try a split fin before a stiff fin ... it turns out this is a better way to evaluate the differences. Just take my experience on this.
By the way I own several of all of the above type of fins, I like them all to some degree or another. But above all else I prefer the classic ScubaPro Jet Fin.