MTH has done some very nice accurate models. The PRR R50B is as good or better than any brass one I have seen. I have about 7 of them all converted to 2 rail, and lowered to proper height. The Fishbelly hopper is also a very nice car and appears accurate. Since it is not Pennsy, I did study it in detail. I do have about 10 of these.
I have nothing against plastic. I think you can get better detail An example is the PRR H21 done by Atlas. It has the rivet and structure detail on both the outside and inside of the body. The brass ones I have seen have very accurate, perhaps better detail on the outside, but the inside of the body when it is empty is devoid of detail, it is the reverse of the rivets and stuff from the outside.
On the other hand, Plastic generally requires larger runs of the same piece to cover the costs of tooling and design to be able to sell at a target price. Brass lends itself to unique models and detailing because of the shorter runs. However the shorter runs also mean significantly higher costs and prices.
