The reasons I bought a Cummins over the PSD (which I was also looking at) are pretty simple.
A little research showed that the Cummins tended to last a little longer on average, and I do plan on keeping my truck that long...
However, I don't rely on the rating of light duty vs. medium duty in comparing the Cummins to a PSD, because the manufacturers are the ones who rate them, and the two used a different duty schedule for their ratings, so those numbers are somewhat 'tainted. '
The Cummins is inherently a higher-torque engine due to it's longer stroke and the inline configuration (a proven combo in all 'big rigs').
The mileage in a Cummins is substantially higher, I get 15-17mpg in the city and 19-22mpg on the highway (unloaded of course).
But what really put me over the edge, was the fact that Ford offers a Cummins in their heavier-duty rigs, F550 and F650, but not in the lighter duty trucks... Why would that be?
About the picture of the rods of each truck, though...
We really have to think about it logically. I do not see a problem with the smaller rods, because their are 2 more of them in the PSD. Sure, the rods in the Cummins are much larger, but it's only because we are making the same amount of power (or more) with fewer cylinders (thus each rod must support more 'weight'). This shouldn't suprise anybody. I just don't like the idea of that picture being our premier physical evidence that our trucks are superior. There is already plenty of that around
Just my . 02