Depends on what you're loooking for. If we're talking the same brand, no DCC differences. As far as model numbers, the GP-38 is a second generation unit, the GP-50 a third generation. What era you're modeling will probably answer that question. After first gen diesels, my expertise gets a little spotty...they all look alike to me. Maybe you could give us a little more info on what you're looking for?
i was looking at the bachmann Norfolk and southern loco just wondering what the differences were between the GP 38 and GP 50 , like power and strength any differences?
For the models there's no difference in power.
Most of the differences will be exterior.
If you're interested in prototypes a quick google will give you an answer.