For 14k lbs you'll want a diesel.
As far as the Ford vs Ram, the biggest thing that pushed me toward the latter was the Cummins. It's simpler, has fewer moving parts and the injection pump is more reliable and cheaper to fix. I did also like some other things on the Ram better, and it's been a great truck to own so far through the first 20k miles, but honestly if you swapped the engines I'd be driving a Super Duty today instead of a Ram. From a towing capability standpoint you'll be happy with any new diesel truck - they're all great in that regard.
$10-12k off is what I'd expect to get off on a new diesel truck.