There's a lot of Kool-Aid in this thread. I have no opinion on the DD rifles other than not liking the way the rails or BUIS look. I doubt that many people have any experience with these guns other than reading about them on this site.
The S&W rifles have been around, gone through some growing pains, and been T&E'd by EAG Training and multiple LE agencies.
Several of my teammates recently purchased M&P rifles, a T, an X, and a standard. All of them had properly staked BCGs and castle nuts and came with the black extractor and crane o-ring. In fact the BCGs looked VERY similar to my LM&T BCG. The S&W parts were put together right. The only thing I don't like about the Smiths is the 1:9 twist, but that will not matter for the 99% of shooters who stick with 62 gr and below ammo.
I would like to see evidence of a S&W Factory gun that came with a DPMS LPK. I have heard second hand (Smith will not say where they currently get their parts, just like most other companies) that CMT and LMT provide the small parts and receivers and that Thompson Center (owned by S&W) makes the barrels. The weapons are then currently assembled in house at Smith. I do not know this to be true, but this is what the Manager of the local S&W LE Distributor told me 2 weeks ago.
I don't own a S&W, I prefer LMT and flavored Colt Kool-Aid, but I think that they are a good rifle that can be found at a great price for what you get.
Either are fine rifles, and today I would probably get whichever one I could put my hands on without getting ripped off.