The New Diesel also has a peaked deck on the stern to eliminate those unintentional stern squats found with the original, as ouachita pointed out below. The bow is also peaked on the new series, which makes it much more adept at tackling hard rapids and staying on the surface.
I have heard many good things about the Mamba series as well and how easy it is to paddle.
You should take into account your own style of paddling when deciding on what river runner to buy. Personally, I am a paddler who likes to have a fast boat that can drive through rapids, skim over holes, and carve into eddies with exact control. Plus I like to catch waves on the fly and try to spin. The New Diesel series fits the bill perfectly for my style.
If you don't like to paddle much, or zip in and out of eddies, then you may want one of the rounder, shorter, slower boats that reward 'drifters' a little more. Good luck with your search and try as many designs as you can before you buy!