You want your truck geared such that when the power is needed the transmission is in 4th gear. If you're able to pull up a 6% grade in 5th or 6th gear at 65 mph you would have been better off with a higher speed axle. With the torque these new diesels make, the six speed Aisin transmission and its high ratio first gear I think I would be debating between the 3.42 and the 3.73. What do the Ram engineers recommend for your GCWR .... Unless you will be towing 100% of the time and always in steep hills why not follow their recommendation?