Unless he wants to do a lot of heavy load towing there is no benefit to at 5600 over a 4500. The first gears and final drive ratios are virtually the same. Both units have a 1.1 4th gear the only advantage of the 6 speed is that 5th gear is an inbetween 1.1 and final drive. Which is really only an advantage if your towing over 10k pounds a lot. That and the fact that 5600s are harder to find and more expensive, I would go 5600.