I believe it's a matter of personal preference.
The K-Line (18" or 21") and MTH UP passenger cars are unique in thier own ways.
The ABS MTH cars have a different 'look' than any aluminum made passenger car, regardless of the manf. They're nicely detailed.
MTH has made a lot of these cars over the years and you can find them easily. Nice used cars can be found in the $35-50 range with a little searching. There was a set sold recently on the forum, 10 passenger cars and two mail cars for under $500! What a deal!
Originally, the value of the MTH cars is what attracted me to them. You can put together a nice long passenger train on the 'cheap'!
Here's the nice UP "Flag" baggage car (they're rapidly rising in cost, get'em while you can):
MTH Coach/Baggage set:
MTH RPO Car:
One last comment on the MTH cars: The DAP UP passenger set did match the DAP PS2 version of the DDA40X about 4/5 years ago. If I recall, there were 10 or 12 cars. Can't find the flyer right now.
The K-Line cars have the 'natural' sheen of painted metal. They have that 'heavy' look to them. Considering when they were made, they're nicely detailed, though not as much as the generic MTH.
K-Line UP passenger cars are hard to find, especially 21" cars. IF you find a them, you're going to pay a premium for each car. Based on condition, $100-$375 per car, depending on which car it is. Most avg $150 per car.
They're stunning though:
Above is the rare, hard to find, flat end observation car.
Here's the list of the K-Line 21" cars:
K4690-40001 | UNION PACIFIC ALUMINUM EXTRUDED Sun Cape Sleeper #40001 21" |
K4690-48001 | UNION PACIFIC ALUMINUM EXTRUDED Dome Diner #8001 21" |
K4690-45813 | UNION PACIFIC ALUMINUM EXTRUDED RPO Baggage #5813 21" |
K4690-47006 | UNION PACIFIC ALUMINUM EXTRUDED Dome Coach #7006 21" |
K4690-40002 | UNION PACIFIC ALUMINUM EXTRUDED PLACID HAVEN PULLMAN CAR 21" |
K4690-46206 | UNION PACIFIC ALUMINUM EXTRUDED CLUB LOUNGE CAR #6206 - 21" |
K4690-45331 | UNION PACIFIC ALUMINUM EXTRUDED CHAIR CAR #5331 - 21" |
K4690-49002 | UNION PACIFIC EXTRUDED ALUMINUM SUN FLAT END DOME OBSERVATION CAR #49002 - 21" |
K-Line 18" UP cars are easier to find, cost less, and honestly, look better on most layouts. I picked a few up to add to my 18" passenger trains.
The Lionel 18" aluminum UP cars are very nice. You may want to consider them also.