That guy you talked with is full of BS almost the same thing that happened to me when I was looking to buy a Colt rifle a couple of years ago from my local dealer.
I talked to the lady sales person but I learned later that she was the wife of the store owner and she told me that Colt was not the best anymore, etc, and I told her that I had been using Colt ARs for 33 years and had never ever had any problems.
She then refered me to her husband and they don’t have them in stock but he ordered me my Colt 6721 tactical carbine and I waited for six weeks before I got it.
I had six former pre-ban Colts started in '71 with my first SP-1 and now currently have two a 6721 that I customized myself with a Global Tactical Operator mid length gas system, medium contour, Rock Creek barrel and a Colt M16A2 upper half with M16A2 buttstock group on my Match Target HBAR lower, the quality is just the same as my former Colts and also no problems.
I have a LMT Defender 2000 16" barrel with LMT M16 BCG, KAC M4 RAS rails, KAC VFG, KAC 300m BUIS, EOTech 551 (which I got from Grant of G&R Tactical) and LaRue QD EOTech mount, I got the carbine about a year and a half ago and It runs fine so far but I can’t tell if it’s as durable as my Colt until I use it for several more years.
The minor differences of my LMT with my Colts is that the LMT doesn’t have finish under the FSB, the lower receiver has a high shelf which is acting as a sear block (Colt have an unmachined web), LMT have small standard FCG pins, Colt have large .170" dia., the LMT buffer tube have 5 position (current production are 6 position) and finished with gray dry lube with three drain holes in the last three rearmost stock detent.
LMT barrels uses Rock Creek barrel blanks while Colt uses ER Shaw blanks.
Same features of the LMT and Colt are Mil-Spec chrome lined 1/7 barrel, M4 feedramps, correct F type taller FSB for flat tops, shot peened/ MPI bolt (LMT are supplied with Crane O-ring for the extractor) and properly staked carrier key screws, Mil-Spec size buffer tube and M4 buttstock.
Overall the forging flash edges of the LMT is slightly rougher than the Colt at the FSB and upper receiver the quality of the lower receiver are about the same except the LMT has a slightly smoother surface finish while the Colt have a rougher bead blast finish.
My two Colts and LMT groups almost the same for me which is around 2" at 100 yards with my Operator medium contour barrel sometimes slightly smaller than 2" using surplus Canadian SS109 ammo.