Sorry, perhaps it was not clear why I commented on 1:29:
I stated earlier, hoping to get an answer:
No one makes the wide variety of USA diesel models that USAT does, so it depends more on what you want to model, USA or European, Steam or Diesel or Electric
So, if he was asking about scale and quality, AND he has been collecting USA diesels, then most likely he will be 1:29 since most all of these models and the rolling stock that go with them are from USAT, all 1:29.
Further advice on what goes well with what would really benefit from what the OP has collected and what he prefers.