I read somewhere that the 100 models were lightweight (as far as carrying and towing capacity) versions that were subject to stringent emissions (catalytic converters, etc) whereas the 150 models were considered a "heavy half-ton" and were not required to have as many emission controls. Additionally, the 150 models may have had a few goodies not available on the more basic 100 models.

My D100 was my grandfather's and it's as basic as they came; the only option it has is carpet. The D150s also had slant sixes as the base powerplant, but they must've had better springs or something because my D100 wallows like a hippo on a slip-n-slide when I tru to carry more than a couple sacks of fertilizer.