If you can afford the diesel, go for it. You will never be happy with a gas truck after you have driven a turbo charged & intercooled diesel. The 6.0 is a great motor, and chevy surrounds it with an outstanding truck, it always seems that whenever you pass another truck and you are pulling 15,000lbs worth of equipment and your only 1/2 way into the throttle, it gives you a little piece of mind and satisfaction.
As for plows, the Chevys with the IFS are not able to carry the weight that a solid front axle can. That is what the manufacters say. I and the people I plow with all carry the same plows as Dodges somtimes heaiver plows in the winter with no problems, then in the summer I have a much smoother ride, and much better handling truck. (and much faster) also with fuel economy, when pulling trailers and most of all pushing snow, I can run 24 hours on 1 tank of fuel where gas trucks are running to the fuel stop all day long filling up.
As for the differnece in the Cummins and the Duramax. I have been a torn man for a long time. I have an undying love for the rattle of the might cummins (IMO one of the most indesructable motors ever made) But I have always been a Chevy man since my Dad's Dad's Dad's Da............... I gave this new Duramax a shot and let me tell you what... that Cummins has to watch it's tail now, because the D-Max is a nippin at it's hind end. (IMO)
I think that you will be happy with either of the diesel trucks, both put toghter a great product, my opinions are favored twards a Chevy, that is strictley due to owning one and not seeing a common rail Cummins that has impressed me in beeing a well rounded truck. They are great 1 purpose trucks (IMO)