My general rule of thumb was always..... fiver over about 13,500 get a dually. Loaded pin weight should always be your guide. Of course its much more complicated than that. Duallies will be more stable when towing heavy. Duallies will cost you 50% more for tires. No answer is as pat as we would like them. Personally other than a under 30 foot fiver I would almost always opt for a dually for stability alone. But then again I dont take my fiver off road either.