I would be curious why they have a used 2014 Cummins with such low miles. Was it a problem truck? Its not as easy a decision as some would have you believe.
If the 2015 can be had NEW with the options, colors and full warranty that the OP wants, its the better truck.
However, if the 2014 is in good shape, they will offer a standard warranty (not 90 days) and the features are in line with what OP wants, its not a bad truck.
I can't believe the dealers aren't more willing to work with you. Sales must be good. When I bought my Tundra, I went through the same process. I sent out several emails and got multiple quotes (and a couple we will match or beat). I knew what I wanted, I knew what I wanted to spend and I knew what my monthly payments would be. First time I stepped foot in the dealership was to purchase the truck. Spent an hour doing paperwork and I was on my way.