I have both the big black Jet fins and the gray Scubapro split fins, TwinJets.
The Jet fins have been through a lot of rough treatment, for more than 20 years. They hold up very well. But they are heavy and cause me to be 'foot heavy'. Also they give me leg cramps. I have 'bicycle legs', that is I used to do rides of over 100 miles occasionally and my average ride length was over 25 miles. When I got those cramps they hit pretty hard because I have a lot of leg strength.
I bit the bullet and bought TwinJets. They require much less force and I never get leg cramps. I love em. The regular Jets give you easier turning when stopped and also allow you to back up a bit. The Twin Jets don't do those things as well, but they do give me freedom from leg cramps.
I got em at Scubatoys or Leisurepro, can't remember which one, but sure am glad I got em. I don't use the Jets anymore, I'll keep em forever though.
When I started we used 'demolition fins', aka 'udt fins', aka 'duck feet'. They are back in production, bodysurfers use them. I might get some for nostalgia. Want some good, high quality, rigorous leg cramps? Just get some demolition fins!