I vote for the F550 all the way.
Around here the Ram 5500s can be had for cheap because they don't sell well. The F550s fly off the lots and they're getting full price for them.
The Ford 6.7 V8 has been getting real good reviews from all my commercial truck customers, very few problem with the new in house engine or trans (most of the trucks stayed the same besides that).
One new feature to research on the new Fords are updated rear leaf springs. They are ALOT longer and have a different rate. The pick ups ride much better and the chassis cabs have a higher GVW.
Also keep in mind the pick up engines and chassis cabs engines are slightly different. Different turbo set ups and engine tunes (all for pick up driveability vs chassis cab longevity)
Two things to keep in mind if you do buy a 550, keep a spare primary fuel filter bowl in stock (plastic, prone to breaking by servicing or rocks on dirt roads) and you'll be doing pads and rotors at 30K miles... sorry, fact of life.
If you do buy a dodge, buy or fabricate a track bar brace immediately. Chrysler and they're infinite wisdom are still making the upper track bar mount (frame side) integral to the frame. Its essentially boxed sheet metal... garbage. I've seen several crack and/or tear off completely from lift kits/tires and bone stock trucks plowing.
Also, as much as I LOVE Cummins in real trucks, this is still a glorified pick up with a medium duty engine crammed under the hood and a boat load of emissions equipment asphyxiating the engine from both ends. There are no more 'half a million mile' engines anymore. Hell, the DPF is only good for 100K ($$$). Sorry... 'nother fact of life.
Good luck with whatever you decide.
And stay away from GM, I won't even start on that topic.