The HD just adds a stiffer suspension in the back so the truck does not squat. With the 5th wheel you do not have the Ability to add WD and help, if you find yourself in that situation you would need to add airbags, which helps with the squat but does noting for payload and numbers.
With that said I would be ordering the HD, but you most likly be a little under your payload or a little over based on a 1500 pin. The HD gives you more flexibility as what many people start looking at, the little bigger one on the lot catches the eye and the fun begins. If the dealer is willing to cut you an unreal deal, which allows you to add the airbag system and some left over compared to the HD, be tough to say no based on your plans.
