Welcome to the forum,
This forum is full of great looking trucks; packed with great mod ideas and there are loads of knowledgeable and experienced people here.
with Rutger34. The Cummins NEEDS to be worked, and worked hard, or it will kill you in maintenance in the long run. If you are regularly pulling trailers up to 5000Lb, the Hemi is all you need. If your pulling 8000Lbs plus all the time, the Cummins is the way to go.....better if you always pull the trailer up hill.
You will never get the Hemi to meet/exceed the Cummins in Loaded MPG, but the cost saving of the Hemi and reduced maintenance cost will far offset this additional fuel cost.
Also, if you want to idle it a lot, stay away from the Cummins. Times have changed, and the gas engine can handle idling much better then the diesel....although it is not good for either one; and not necessary. Idling more than 3 minutes is strictly for the comfort of the human.
Enjoy the Z!