I don't have any of the vintage Athearn or Mantua metal cars... yet.
BUT, I've taken a fancy to Varney's tinplate cars. They look surprisingly good. Just recently I purchased these three Varney's:
I have already fitted Kadee coupler boxes and Kadee's to the PFE and ATSF car, as well as installed steel wheels and attended to "issues" that I saw. The MDT reefer is going to take a bit more work on account of the pot metal coupler pocket. These look plenty good enough to run among newer plastic models of such era equipment. I was specifically impressed with the sharp printing on the sides which is better than found on Athearn "Blue Box" injection molded cars!
I still find myself checking eBay for some deals on more Varney metal cars.
Andre