It shouldn't be a 8K difference between the 6.7L Cummins and 6.4L HEMI. IIRC, the Cummins was a $6700 option on my 2500. The standard engine is the 5.7L HEMI. Now this is true for 2014 Ram 3500s as well. Being that the 6.4L Hemi is around an $1800 option over the 5.7L HEMI you are really only saving approximately $4900 to upgrade. Either way that is a lot of money but IMO well worth the added cost. I was planning on going with the HEMI but the power of the Cummins and the exhaust break for going down the mountain passes is absolutely wonderful. I would pay the extra money if put in your position.
Best of luck to you.