What is the difference between their High Speed NM DMR trigger and their High Speed NM trigger? I am buying another general use trigger but want something better than the SSA-E.

It's the same trigger, it just has a different second stage spring. When I bought mine from Rainer Arms it came with all the springs so I could set it up as a DMR, Match or Service Rifle trigger. I started out with the Match spring but found them to be just a little too light for my taste so I moved up to the DMR springs and run it at about it close to it's lower setting, around 1/2 lb (second stage.) The spring rates for the three springs over lap but the DMR second stage spring just had a better feel to me. I'm not sure if you get the extra springs from other venders or not but Rainer Arms includes them.

Tweaking the the 1st stage spring makes a huge difference too. Every bit as much as the second stage.

These triggers do require a little more attention during the install than most. Once set they are fantastic though.

Here is an install guide from the man himself. I found this video a little more helpful than the printed instructions.