I run both 21" and 18" (no MTH 17 1/2" passenger cars, but I like their 70' baggage cars). The only advantage I see of 18" cars is you can get a few more of them for a certain length making the train look longer. Other than that, 18" cars look short behind most of my scale engines (they sort of look okay behind the K4s).
I like Jonathan's comment that folks will run huge articulated locomotives on O-72 but don't like 21" cars on O-72. I do agree that Lionel's kinematic couplers are not the best design, kind of like their freight trucks but maybe not as bad. Luckily I only have one Lionel car with these (outside my AFT), the rest of my 21" cars are K-Line, GGD, and 3rd Rail.
When I run a passenger train, it is either all scale or all 18".
