You are comparing two of the better diesel trucks out there. Have you looked at a regular cab to solve the length issue? These are dirt cheap used.
I wouldn't run any emission strangled diesel that doesn't use DEF. The older ones used too much EGR to control NOx and plugged the DPF's up way too fast costing lots of fuel to burn them out even when working hard.
We compared them new in 2008. And watched our broker run many of GM's towing 100%.
The Dodge wouldn't STFU over the seat belt - This supposedly is a free country...
The comfort of the seats was like sitting on plywood vs the GM seats.
The passenger side floorpan on the Dodge has an uncomfortable hump in it on the left side.
Historically you can't beat the Allison transmission. Frankly everyone else has had to catch up to it's reliability. The known reputation for Dodge transmissions behind a diesel was deservedly poor at the time, 2008.
Looking back on the decision to go with GM:
RAM/Dodge has completely kicked GM arse around the block with cooling! Frankly GM writes power checks that the cooling system can't cash. The cooling system includes shifting down to a lower gear to rev up the fan and water pump instead of lugging it in double overdrive. IF GM is still using the Obsolete Spring thermal fan clutch vs. the modern Electronically controlled fan clutch like Cummins does... The Obsolete style overheats the AC system as the cold radiator cools the air off before it reaches the fan clutch when you need it the most with a hot cabin. Cooling need is also controlled by letting heat out the exhaust where a turbo with excessive drive pressure or excessively heating the intercooler makes a difference in cooling ability.
Let me put a fork in GM cooling: You know the video of a first gen Dodge leaving a 2005 LLY Duramax in the dust because the Duramax overheated and shut down? Even my
modified 6.2 turbo would out run a 2005 Duramax towing Death Valley! Sitting on the side of the road waiting things to not cool down in a survival situation and wanting my proven 6.2 GM diesel (already ran the 6.2 through there towing) because the 6.2 would be moving with the AC on is about the lowest point with a Duramax. Easy to do when the 2005 is on the side of the road with temp gauges in the red. Yeah, over not enough cooling by running the turbo past it's choke point in the tune. Hard to cool anything when the intercooler is like 500 degrees. Many other reasons 2005's had shut down overheat cooling issues. Note year specific the GM 6.5's needed major cooling system mods and would also be burnt up without mods.
Our 2008 Duramax lugged double overdrive towing so badly normal ECT to us was 235. You had to manually downshift on light grades to run sane ECT's. It ruined 250 degree rated oil cooler hoses on an aftermarket oil cooler kit. It cooked the passenger side battery off in 88K miles at 1.5 years old. 110K it split a heater hose. It ruined engine oil due to heat and would get oil so hot we would get shutdown now alarms over less than 9 PSI of oil pressure. GM has likely buffered that alarm since to ignore the condition.
The Fuel filter primer system and lack of a lift pump was always a PIA. No lift pump to fail, but... Join the crowd for "useless" diesel filtration and water separators.
Change a water pump just once on a Duramax. I DARE YOU!
Yes they don't fail as often, but, when they do it isn't 2 bolts. You pull the engine damper...
Same with the FCA on the CP3. Easier on the Cummins.
GM wiring for HVAC and other things like heated washer fluid is notoriously too small.
Brakes are better on the RAM/Dodge as the GM brakes would get noisy and fade when pushed to the limits on The Grapevine.
How exactly are you supposed to change the driver side headlight bulb?
It involved a saws-all last time I did on this era GM body style due to a support you couldn't get to the bolt on without a major remove most of front clip job.
We had good luck with our 2008's emission system being trouble free. Thus with the comfort, emissions system, and Allison it was a better choice than the 2008 Dodge.
We would deliver parts out of a 28' full height cargo trailer. After pulling a long grade into town the damn thing was always in regen in town. Due to using crazy EGR to keep NOx down even a full power pull wouldn't burn the DPF clean. This is why I recommend the newer DPF emission system that doesn't need so much EGR. Deal on truck or not, well this is why the non-DEF years are so cheap...
Attention: TDR Forum Junkies 
Oh! I just remembered that it was the same truck I mentioned earlier :-laf This particular unit and ALL of the others suffered from worn suspension parts due to running on mostly dirt roads for many years! This one as been good to my company so far, but I think that it has been thoroughly used up and won't live much longer. 
