Hi tbg:
I hope i can help you out because i'm in the same boat as you. I'm also a looking for either 2 or 4 mm compressed neoprene drysuit,except this one is made by Diving Concepts. I two was not sure which mil to go with,so i e-mailed Diving Concepts and this is what they said.
For mobility go with a 2mm for the fact that this would be a more trimmer fitting suit and with a drysuit you have the option of what type of insulation you want to wear so the insulation difference between the 2mm and a 4mm becomes minimized.
As for durability of the compressed neoprene they commented that they use a laminated w/ a heavy-duty nylon exterior.
I know that Diving Concepts use the Rubatex G-231 which is a better grade of rubber.
With the 4 mm you sure would have to use extra weight in which i try to stay away from.
Diving Concepts also mentioned that they have a hell of alot of these suits out their with hundreds of trouble free dives.
I hope to get around to order mine in the near future.
I seen the website for Northern Diver and it seems like their suits are in the same line as Diving Concepts. Very good....
Let me know how you like yours when you get use to it.
Hope this would help you out somewhat... Safe Diving...