100days-a-year:I use SP TwinJets,love to see someone be able to swim into current as fast or catch me on a long swim back to the boat wearing longblades.So far it hasn't happened,then again I'm not old or out of shape so kicking fast doesn't cramp me up.Long blades work great for freediving as they require slower movement which uses less 02 outta your bloodstream.On a small to medium boat they can really get in the way,on a bigger boat you don't have tight clearances.This could turn into a "my dad's better than your dad"quick as it is real subjective.
I own SP Twinjets, SP Jetfins and Cressi Gara 2000HFs. My experience is that the Gara's are a little bit faster than the Twins, but not much. Either fin is pretty good for covering ground, but either one takes a minute to get going.
The Jets are instantanious and powerful, and require very little movement, which I like for approaching skittish stuff. They don't go fast, but they're quick.
The Twins are pretty fast but require lots of movement. This requires lots of breathing, at least for me. They are extremely comfortable, it feels much like kicking in bare feet. It can be hard to get used to that.
The Garas are hard to manage topside, not much faster than the twins (for me anyway) and take a while to get going. To me, the feeling is similar to jumping on a trampoline, where you kinda time your bounces to build on one another until your flying 20' up in the air with each jump. Once you're going though... you cruise pretty nicely without a whole lot of effort.
There is always some kind of give and take.