I've been agonizing for months over which to buy, but finanlly settled on the DDM4V3. I'm not confident about the piston system simply because there is NO standard as yet defined by the market. As such...if I'm in the middle of IRAQ and need parts for my piston driven AR/M4...I'm S.O.L.
The Daniel Defense (which I bought 2 days ago) is completely compatible with everything...and is a snatch and grab system that has all the lower compression advantages seen in mid-length gas systems...and is also universally the same as the rest of the world as far as the AR/M4 platform.
I spent some time over the phone getting to know the Daniel Defense guys and they were rock solid in knowledge and weren't boastful or anything...in fact they were quite the opposite...humble and friendly. Not your typical gun-snob (since I know relatively little about being an armorer).
Having spent 8 years in the Marine Corps as a Sergeant, I only used the M16A2 because it was the dominant rifle at the time I was in through 2002. The M4 Carbine was just becoming "mainstream" in the military at that time because of MOUT (Military Operations in Urban Terrain) and the quick draw aspect of the Carbine for house to house and close quarters fighting on the streets.
All in all, I've spent the last 6 months researching and getting to know both guns pretty well. I DO like the look and feel and heaviness of the Ruger...but for battle purposes it is a bit clunky and I do worry about the compatibility issues of piston driven systems vs. gas-tube systems.
Good luck with your decision and I hope that my opinions offer some council to anyone who is considering piston systems vs. gas-tube. One other thing...I felt about Daniel Defense (and seemed to be echoed in every gun shop I've gone to) is that they are considered to be a top-shelf rifle..the fit and finish is immaculate and the 1:7 twist is actual mil-spec and provides additional accuracy that the 1:9 twist guns do not. Likewise, Daniel Defense ships with a STANDARD FULL AUTO bolt carrier group which will eliminate any fears of "bolt crack" that may come with the semi-auto bolts like the Ruger has.
Anyway...that my 2 cents for what it's worth. Sgt Chris Hammond