in stock items are always "great guns/great buys" from the dealer's perspective. They're in business to sell guns, and they have more incentives to move existing stock than to add more. As an unabashed capitalist, I've no issue with that, but it doesn't erase differences among brands.
As regards the commercially configured M4 platform (i.e. semi auto, 16" bbl), Colts are built to certain QA/QC and material standards (the TDP); S&W to somewhat lessor standards, and RR (along with the other hobby brands) to even lessor material and QA/QC standards. Those standards may or may not be relevant to you and your needs, and if the QA/QC and material standards are not of concern, and the elevated risk (real, if not always readily quantifiable) of a functional failure associated with adherence to lessor standards is acceptable to you, there's not much reason to be picky abut brands.
However, if you want a rifle suitable for potential hard use, the regular M&P 15's (not the Sport) are considered by some trainers and hard users to be the minimal entry level carbine, above which would be Colt, DD, LMT, BCM and Noveske. The M&P Sport is a lessor gun than the regular M&Ps, but I've heard nothing of S&W using inferior quality parts across the M&P line, and frankly do not trust that info. Other than the semi bolt carrier, the bbl steel and a few other items (none of which seem to show up as problematic in high-round count class AAR's) , S&W has consistently built a solid gun once they started making them pretty much all in house some years back.
My journey has began with Bushmasters; 4 of them, actually, 2 of which had serious issues and none of which was assembled properly. I then migrated to a mix of Colts and BM's, then Colts and M&Ps, and now all Colts (save my M&P 15-22). While I would readily consider another non-Sport M&P, Colts have never been more affordable in recent memory than they are now, and Colts' the company is presently more consumer-friendly than has been true in many, many years. If someone wants a carbine suitable for hard use in a defensive role, I do not see any reason to step down to a BM, Windham, DPMS, RR, etc. in the current market when rifles made to higher standards are so readily affordable and available.