I recently upgraded from the Twin Jets to the twin Jet Max. The biggest difference I can tell is the foot pocket. On the Twin Jets it's very hard and wears on the top of my foot after several dives, but on the Max's the pocket is VERY comfortable... It was worth the switch just for the improved foot pocket.
As far as the propultion, they both seem to be about the same to me. I notice little to no difference at all between the two of them.
If the pocket on the Twin Jets was not so hard, I would have never switched, but I'm glad that I did. just wish they had springstraps for the Max's....