I don't know exactly what you are referring to, but the main difference should be the lettering in the letter board, i.e. prior to 1946 it would be "SOUTHERN PACIFIC LINES", and post 1946 the letter board would reflect "SOUTHERN PACIFIC". Thus, it depends on what era you desire to model.
For what it's worth, my local hobby shop has MTH SP Daylight cars, and the steam locomotive in stock. Contact Berwyn's Toys & Trains at (708) 484-4384 for further details.