I want to upgrade my Midlength rail system to something top tier. I cannot decide on the Daniel Defense or the LaRue models. I was leaning towards LaRue because I have some of their other products and have been pleased with them, but it is a little heavier than the DD 9.0.

Is there any provision on the DD unit that keeps the barrel nut from backing off?

I realize this may come down to personal preference, but all things considered, what system is better?

In my opinion, one is no better than the other. They are both at the top of the FF rail food chain.

Larue has the anti twist clip and uses an epoxy to hold the two pieces together for a cleaner look. Slightly heavier design, maybe more of a oval feel depending on rail panel.

The DD has lighter weight, and uses a couple spot welds to hold the two pieces together which might be stronger. No way to guarantee it won't twist, and a rounder feel depending on rail panel.

I own one of each, I wanted a Dissy look for one of my rifles, so I went with the Larue 13.2. Then I wanted a light weight rifle, so I went with a DD 9.0.

Quoted: I want to upgrade my Midlength rail system to something top tier. I cannot decide on the Daniel Defense or the LaRue models. I was leaning towards LaRue because I have some of their other products and have been pleased with them, but it is a little heavier than the DD 9.0. Is there any provision on the DD unit that keeps the barrel nut from backing off? I realize this may come down to personal preference, but all things considered, what system is better?

If a few ounces in weight is extremely important then get the DD, if not then go with LaRue.  

Talking about 7.0's, but otherwise the feedback is the same:
www.jobrelatedstuff.com/forums/topic.html?b=3&f=118&t=262445