Well from the research I did over the last year looking for my new truck here is what I came up with. This is from personal observation, from reading and from word of mouth on the vehicles they drive not heresay.
Dodge: the Cummins.... what can you say really. Great engine but people complain about the auto transmissions. Go read the tons of Dodge forums.
Didn't have the raw power that the new Ford and Chevy have.
GM: I like the GMC better than the bow tie, I think it looks better. Cab forward I love the look, bed back ..ugh. The Urea system doesn't seem as well designed as the Ford from what I read on thedieselplace.com. There were too many complaining about that system messing up.
I drove the GMC Denali 3500HD and they've done something with the throttle on these things. The torque management made the truck feel underpowered although I know it isn't. Even in the Denali it feels exactly like I was in my friends '07 Suburban. Overrall I really really liked it, a lot.
Ford: It felt very comfortable to me, it was new yet familiar. Finish on the interior was good. The towing mirrors are great, the new information screen is real cool, the new gauge colors are easier on the eyes. Dash layout felt good, easy to reach everything, tons of power points and cup holders.
Right now in the game I do believe Ford has a superior product with the new Super Duty's. I consider myself a very fair person concerning this age old battle but it's like discussing politics or religion.
So to my experience with my truck. I have put 6,000 miles on it since late June. 4,000 in the last 2 weeks. The transmission issue some are mentioning (from what i've read since i've never experienced it) is only when it's empty. I haven't read anyone complaining about the tranny while towing. I know I can't, it did it's job and got about 11.8mpg at an avg of 75mph with a 14k trailer.
I did have to refill the urea, no big deal, open the blue cap near the green cap, pour in urea. Changing the oil was surprisingly easy as well. I don't have a problem with the trailer sitting nose high.
For mods, don't touch it, at least not now. It is a lot of truck as it sits, seriously. The stock air box is rated at 500HP, which means you're good. I use the regular filters because K&N are just a waste of money, my 0.02.
If you want to really do something to this truck then the warranty will be gone so don't spend the extra $2000 on an extended warranty just go ahead and buy a 5" exhaust and an H&S Mini Max programmer. This will give you the throaty sound, the whine of the turbo, another 150hp + gobs more torque and you won't have to worry about Urea refilling or DPF regenin'g BUT AGAIN.... kiss that warranty bubye.