This is actually a tough question for me to answer........
At first I would say "modern", but with each year I spend in this hobby, my appreciation for other eras is increasing. I would have never thought of buying anything steam driven, but now, some of those transition era steamers look pretty great and might find a home on my layout. There is a brute strength to the steam era that is unmistakable. And I think when modeling the steam era, there's a Norman Rockwell feeling that comes with it. Fedoras, suits, skirts, big metal cars, and everything built like a tank (at least it looked that way). A simpler time, not quite as fast paced...but I'm sure everyone that lived in that time would refute that observation. With the advance of the diesel transition era, there's a clear nod to technology but with an elegant flair. The modern era, which I mostly model, is more utilitarian. I agree that there's very little difference between the design of most modern diesels. Variations on a theme, but no real major design differences. Fortunately, there's more variation in passenger diesel design than anywhere else, but it's not easy to incorporate either the high speed rail and lower speed passenger service offered today on a typical home layout in Nscale.