I do not own any 21” Passenger cars due to the lack of 0120 curves on my clubs layouts. I do have a set of Lionel 18” Aluminum cars (PRR), 18” Commuter coaches PRR and a pair of Kline 15” US Army passenger cars for my military train. All of these perform as expected on my clubs layouts where the maximum curves are 072.
I do not own any MTH 18” passenger cars but have pulled friends cars at both clubs and although very generic have not had any operating issues. I do not plan on buying any new Atlas (MTH) cars now being offered as I feel the only improvements Atlas will be making is raising the price (which is their right to do) but if I do have the desire for a MTH 18” Premier set I could fine them on the second hand marker without too much trouble.
In short my preference is 18* passenger cars.
JohnB
