Good choice on the Cummins. I had a 2011 2500 Hemi for 18 months, 40k miles, and I couldn't handle the towing mileage or lack of highway power. I tow 6k five days a week, and was getting around 7-8mpg with my 7x14 enclosed. I had to kick it down to 3rd quite often just to maintain 55-60 mph.
The 2013 Cummins is a dream to tow with, heads and tails above the Hemi powered truck. It also rides way better, not sure if that's a 2013 difference over 2011, but it's a huge difference. As I'm sure you've read, fuel mileage is awesome on the 2013 Cummins too. I'm spending much less per month now because of the fuel savings, this is after a $150/mo payment increase.
Also, DEF usage is about 1000 miles per gallon for me. This is towing almost all the time too which uses more DEF.