I don't know much about them but I would expect that the simpler, the better.
I had a 518 a few years back that was a respectable rebuild using Cummins spec internal parts but it was the electronic (46E ?) version. I didn't want to deal with a stand-alone controller or manual valve body. The lower 1st gear of the 500 is a better idea in my opinion. One could run a taller axle gear and still get good acceleration from a start.