I would without a doubt go with the 5.9 even with it's less than desirable transmission that requires band adjustment and meticulate care just to have a decent life span. I would try to get one with close to the < 07.5 MY as possible as I believe Ram finally improved the fuel filters which should help with injector life, which has been an issue to some extent.
Stay away from the 6.7 with the DPF unless your truck will be a highway runner with little town driving/idling. This configuration with it's soot choking EGR will bite you with your wallet in the cross hairs! Also fuel economy will be less with this configuration as well.
Good luck...