I think now days, you buy the diesel for the power that it will provide. The fuel economy that we used to look at, is offset by the higher costs for the fuel now.
The longevity thing isn't what it used to be, because both engines will outlast the use of the vehicle in most cases now days....we simply don't keep them as long as we used to.
How much more will that diesel cost than the gasser? Could a year older truck balance the price differences?
Personally, I'd go with the diesel, with that trailer, but then again, it will depend on how much he's actually towing it.....and will it be a daily driver.