Re: which upper would you get?
Both are good. I consider the LaRue line to be in a different catagory tho', a cut above the DD. The LaRue has a better barrel that will normally prove more accurate. Had an 18" Larue Stealth with a 13.2" rail that's beautifully made and accurate (traded to a friend) and just got a LaRue DMR-16 to replace it that I haven't used yet. On the flip side, I bought a stripped LaRue upper receiver to build a 3-gun rifle on and discovered that it was "dimensionally challenged" in the charging handle trench thereby affecting the tracking of the gas key. Worked fine during dry fit but wouldn't chamber or run at all. They made it good. Great guys, great service. Everything they make is first class. Love their scope mounts.
I do believe that Daniel Defense's M4 rails are the best out there, though. Vltor's uppers look to be first rate but I haven't yet used one personally. Their stripped upper receivers are probably the finest made and am currently running one on my 3-gun AR.
Have you considered either a Noveske or an LMT MRP (gas or piston)? Absolutely wonderful hardware. Hard to go wrong with any of these, really.
As far as POF goes, I've heard from both ends of the spectrum. Seems there is quite a bit of inconsistency rifle to rifle. Lots of good ideas, but I need accuracy and above all, reliability. I'm not sold.