Of the GP38-2's that are available, the Proto 2000 one is the best. The detail is excellent and the drive is very nice. Apparently some of them have electrical problems, but none of mine have had any problems.
The Atlas Trainman model has a great drive system, but the fine details like grab irons are missing. For the price, it is a great deal, and all the grab irons are marked, so drilling and adding your own is easy.
The Athearn RTR model is like the Trainman one, no finer details, but the shell is not as nice and neither is the drive.
If cost is the main factor, I'd get a Trainman GP38-2, but if the cost of the Proto isn't an issue, it is the more detailled model.