My very personal take on it: I started with TwinJets. I bought them because the shop owner told me they would be less likely to cause cramping. He was right -- I never got a cramp while using them. They were very easy on my legs.
Then, as the result of a class, I bought a pair of classic Jets. The first time I put them on, I thought I had concrete blocks strapped to my feet! But then I went to do the various kicks I was learning in class, and found them so much easier to feel and do that I was amazed. I had thought the difference was mostly hype, but I was wrong.
Diver0001 will tell you that divers of skill can do anything with almost any equipment, and he's right. But for beginners, some things make certain jobs easier. Twin Jets are great for cruising, and easy to learn to use. But to generate their power, they move a LOT of water, and I was watching my husband's fins kick up the bottom from about six feet up yesterday. Jets are heavier and stiffer and more work (and yes, I get cramps occasionally with them), but they do permit a novice diver to use more precise propulsion techniques more easily.
