Basically the same weight, same axles (excl brakes), same hp. With all that, the ability to go thru mud would depend more on tires and the driver.
In defense of the M939's (since I have one, lol), I have to admit I didn't like the idea of an automatic at first, but wanted the full air brakes and non sprag t-case. Now that I have it, I have to say its fun driving the auto in a big truck, especially when in town. With the deuce in traffic, I will take off from a green light and work away through the gears just to get stuck at the next light and have to do it all over again. With the auto, you just mat it and go. Pros and cons to each.