I have the Neosport 3mm I started with. I glued on a thigh pocket and it has served me well for 5 years. It has a few shallow tears and can smell like a biohazard but still does everything it did on day 1. I recently found the sales on Deep 6's websight and pulled the trigger on a new D6 3mm.
Specials
There is a difference. The D6 is definately warmer, is lined with an almost fleece like material, has material that seals around the ankles and wrists, and came with 2 thigh pockets already sewn on. It's reinforced at stress points, and has Kevlar? On the knees, crotch, and shoulders. It flows almost no water. On a recent Cenote dive where most people dive at least 5 mm or layer a 3mm shorty over a full 3mm, I dove the D6 with a hood and was fine, and even took the hood off on the second dive. I don't have a lot of dives on it, but even new it did not take much more weight than usual to become negatively buoyant.
I think I paid $80 for my Neosport in 2014, $40 for the thigh pocket, $16? For aquaseal, and the suit is great for tropical dives. The D6 was $300 with 2 thigh pockets built in, a rash guard, and a pair of gloves.
There is a difference, especially in warmth, comfort, utility, and what I believe will be longevity. Whether it is worth $200 difference is up to you. I'm glad I bought the D6 and will recommend them to the students I work with. I'm going to add ankle zippers and a pee zipper and I think it will be perfect.
Do you need it? Probably not, I still have my Neosport 8/7 for Chilly Willy dives, but the D6 will be my goto suit for most dives.
Good luck,
Jay