Well, I just went through the same thing.
I had a 2004 Dodge 2500 with the Cummins. I already went through 1 tranny and was enticed by the GMAC emplyee financing. I went to look at the Chevy's and figured, what the hell, let me check out the Fords. First let me tell you, I'll never go back to a gas truck. I love driving diesel, and now especially that is is roughly $.50/gal cheaper than regular, with better mpg to boot.
I have 2200 miles on a 2005 Ford F250 SD FX4.
From my observations, here is what I came away with.
The Cummins is probably the best engine of the three.( Dodge has the weakest trannies of the three)
The Chevy had the most "car like" ride to it.
The Ford had the biggest, most refined interior. I also liked to looks the best.
One other thing i didn't like was that the Duramax had aluminum heads, which I don't feel are conducive to longevity.
The final selling point is when I compared the front ends of the Chevy and the Ford side by side. The Cevy has a independant front, where the Ford has a live axle, which looks about 10X beefier.
I get about 16mpg city and about 21mpg hwy.
If you have any more questions please ask.