the NT is the top end of DYEs line.
It has a different operating system with both a variable volume fire chamber and variable air-through-bolt flow. it allows you to fine tune the markers performance, efficiency and feel.
The DM is the next step down and lacks the tuning option of the NT but is still a phenomenal performer.
there's a sort of comparison here:
http://dorosports.wordpress.com/2011/01/31/dm11-vs-nt11-which-one-is-for-you/