I am a huge fan of Lionel;s 18 heavyweights and extruded aluminum cars. I have a set of Southern Crescent cars and the outside paint finish is great and the interiors are some of the best I have seen with different color seats, seperatly applied toilets and sinks and a variety of people. There extruded Aluminum cars are also great. I have a set of the SF cars from around 2005 and they also have nice interiors. I also like the 18 inch size better that the 21 inch cars.
I also have a set of Golden Gate Depot 21 inch extruded aluminum Empire State Express cars the have a nice stainless looking finish and very nice interiors. GGD cars are also nice because every car has different window configuration which looks nice running on the layout.
I expect to get some grief for this but I am not a fan of Kline's extruded aluminum cars. I find the interiors boring with white floor, seats and the holes in the floor. They also seemed to use a softer grade of aluminum because they scratch easier.
I also have several sets of MTH 18 inch modern cars and while not as detailed on the interior as Lionel;s, I do find them acceptable, especially for the price.
Of coarse I am speaking of offerings from the last 15 years or so because I have no idea what the cars coming out in the near future will look like.
Good luck on your decision and happy railroading,
Don