I would say for your use, I think you would be happier with the gas and an automatic. Just make sure you get the tow package this should include an additional oil cooler, a bigger radiator and should also include an additional Trans cooler. It also helps to have 4x4 
The Alison Transmissions are pretty bullet proof provided (a) regular maintenance has been done (b) it's never been over filled and (c) the proper fluid has been used. (D) It has never been abused. But that goes with any transmission. 

Also, the best place to find out about transmission quality is a reputable transmission shop. They see everything