First thing, your FFL is thinking about his pocketbook and not giving you good information, the only time you run into big time fit problems can be when you include Colt components, everybody elses components and parts are pretty standard. Normally there aren't any fitment problems, sometimes the upper and lower are a bit too tight, but some examination can reveal where the problem is so you can fit the parts together.
To give you a "for instance", I've built 3 from kits, none of the 3 had any real fit problems, the first one was pretty tight between the upper and lower, but you could put them together, you just had to use a little pressure, when I was modifying that first AR (the one that was already tight), I went to put the new upper on the original receiver, it wouldn't go together, the rear lug wouldn't fit into the recess in that receiver, I pulled out one of my other AR's and tried the new upper on it and it fit fine. I'm comparing the receiver that fits with the one that doesn't, the first lower receiver was a Rock River, the second one a Stag Arms, now CMT makes both receivers, but the Stag has a relief cut in the slot where the rear lug on the upper receiver fits, the Rock River didn't, some additional looking confirmed that this was the case, so I took a Dremel and opened that slot up a bit, and it fit fine. What was the difference between the 2 different brand lowers that had both been made by CMT? Apparently Rock River has a slightly different spec for that area, Stag had observed that area was a potential tight spot and made theirs a bit bigger to eliminate the potential problem.
I do want to emphasize that this is unusual, none of the others had any fit problems at all.
If I was going to recommend one over the other, the stripped lower from Stag Arms fits better out of the box, I think Rock River orders their receivers a little tighter, they are thinking about a tighter fit between the upper and lower, and rely on their assembly people to adjust them for fit. I might add that upper to lower receiver fit on an AR has NOTHING to do with accuracy.