I have had a 6.7 ford diesel and currently running a 6.4 hemi. I go between MT and CA. 
It does truly come down to personal preference. 
 IMO diesels always handle better on the road than gasser's especially on those long windy icy hwy stretches. I like the heavier more planted feeling. 
In the Winter months diesel requires some extra thought ( plugging it in and winter mix). If your towing you cant bet a diesel . 
 I think for general use, towing medium loads, and off-road is where the gasser shines. They also generally cost less to maintain. 
 I don't know anything about Chevy. 
I can say, a ranch boss I work with  just got a brand new GMC AT4 3500 and it is unbelievable nice. The truck is mainly a tow rig for his / daughters horses though.
 I am not sure what level of truck you are after. It gets pretty nit picky between the brands. I love that Ford offers the fx4 package ( you get a rear locking diff) as an add on package to all truck levels.  I also love that you can add a power wagon package to a ram tradesman. 
I like the ram seats , entertainment system, cup holders more than ford. I also have a lever activated 4x4 system in the ram ( which is I just think is cool , not sure how it holds up to constant active / inactivation yet). 
 In the ford , I liked the seating position better ( easier to see over the hood on an incline). I actually prefer the overall look of the ford truck. I think the interior plastics seem to be a bit higher quality. 
The ford 10 speed trans is very smooth. 
Hard to beat a hemi though. 
Like I said it gets nit picky . I think between Ford and Ram its really what feels best to you. 
 Lastly, Wildfire , Forest Service , BLM , and USDA  predominantly use Ford and Ram gassers.