I have a 2016 Dodge Ram Cummins with the 6 speed auto and I can say without reservation that it's the best transmission I have ever driven. No tonly that but it works in tandem with the tow haul mode and the engine brake to make descending long steep grades a breeze. 
The 6 speed will downshift automatically when needed and work with the engine brake. You can set the engine brake on FULL or Auto. Auto will let you set the speed you want to descend a hill at and it will keep you there.
And the fuel milage will be much better with the auto trans than the manual.
By all means get the auto.
By the way I have tested mine from the Baja to Alaska and back this year so it's seen a bit of everything.