Both are great trucks and I think you will be happy with either. Ford has had more issues with their diesel platform in the past, that said their 6.7 seems to be a solid workhorse and SHOULD provide many years of service. Ford has had the edge in fit and finish in previous years, that said Ram has REALLY stepped up their game on interiors, fit and finish, etc.
The Cummins/Aisin platform is a proven workhorse. Cummins engines provide power for boats, tractor trailers, firetrucks, school buses, heavy equipment, etc. Cummins are good at what they do. The Ram Cummins is designed for a 400,000 mile service life and with proper maintenance they have been known to go 1,000,000 miles. Keep fluids changed, keep it lubed, keep it cool and it should last for a long, long time.
Personally I would be smiling ear to ear with either new truck. That said I'm more of a Ram guy myself. Hard to say any new vehicle is a value nowadays, but you can get some really good deals on a new Ram.