In the pecking order, it is just an A gasser that happens to have a (Cummins ISB) diesel engine. The rest of the chassis technology is A gasser.
In the years these FREDs were produced, I would prefer the gasser, because the alternative GM and Ford . gas engines were more quiet at idle and cruise, a bit more powerful than the Cummins in the rare situations when peak power was needed. However, when you stomp on it to demand peak performance for a while, the Cummins diesel will top itself out at 3000 rpm, the GM up to about 4000, and the Ford a bit over 5000 rpm, which can be pretty noisy in an external cooler kicks in.
People who have the Freds most likely prefer them, it is parallel to the gas vs diesel argument for passenger cars today.