Since you said "scale" that implies you want an accurate scale model of the State of Main car if possible.
Atlas O 1937 ARA boxcar was a very common car and the model is a pretty good representation of the prototype.
I don't what MTH uses for their car. Lionel as far as I know has only their PS-1 (Pullman standard welded side car) in a scale size car. They also have the old Weaver line which is based on the same prototype.
I personally like Riveted cars such as the Atlas model better than welded side cars. All the little rivet heads just add something for me.
Lionel Round Roof cars are scale, but were probably never owned by those New England RRs. the Atlas/MTH woodside cars both outside braced and double sided, are probably too old prototypes to have ever been in that scheme.
so with the little I know, I would lean strongly toward getting the Atlas O 1937 ARA boxcar.
