We use both(rockets & jets)....the schools(US Army CDQC, USAF Combat Diver, USMC Combat Diver, BUD/S phase2) phased-out the rocket fins & replaced them w/ jet-fins. I use both my old issued rocket fins & jet fins for training. When you get to an operational unit, you pretty much buy what you want (it's a privilege for special-operational units)...some guys in my team use power fins, some use turtle fins..although we all have rockets & jets. We also have different fins for different purposes like water jumps(both planes & helicopters) like bio fins. Most of the time it's operator preference really. When you're going through the different courses as a trainee you will be using the Jet Fins. If you want nice easy fins, don't use these coz a lot of people don't have enough leg-strength built up to use them for a long time (unless you know you're strong enough of course). When you get used to them, they're not bad at all.