My son is considering a 2500 for his next truck. What advice or suggestions would you offer? Possibility of towing someday such as a future boat and/or hauling his wife's jeep to off-road events. Has a car for work so the truck won't have to be a daily driver. Gas vs Diesel? Options?
Either engine will probably do for your son, but to better answer your question we need to know what he is mainly looking for in a truck and his motivation behind buying one. Cost, performance, reliability?
If it is cost, then the 5.7L will be the least expensive in total cost of ownership by a few grand. He may be willing to pay for more performance in the 6.4L or 6.7L. Whether the 6.7L or 6.4L is the cheaper route will depend on many variables like fuel prices, interest rates, local depreciation values and such. For me, a 6.7L is actually less expensive in total cost of ownership and I would still pay for it even if it were a few grand more regardless if I towed or not. However, this is not the case for everyone and he may or may not willing to spend it for the added performance.
Reliability is a mixed bag. The Cummins may have emissions issues while the 6.4L seems to have lifter issues with the MDS. The only one that seems to not have hardly any issues is the 5.7L, but that may be due to how many are out there compared to the other two. Regardless, none of them seem to have any major issues warranting any fear of not getting one over the other due to reliability. Unless you are one who is just looking for a reasons or justifications of getting one over the other. In that case you will find them.