You are asking about dual rear wheels (DRW) not dual axles like a semi truck.
Here's my opinion-
DRW- more stability I've towed with several single rear wheels and a couple of DRWs. The DRW feels much more stable on curves, stopping and quick maneuvers.
Another advantage is extra tires in case of tire failure. You'll be able to limp off the highway to a safer place to get tires changed.
8ft bed- more room to carry the extra stuff. In 20 years of towing a 38ft 5th wheel I have never been in a spot where a long bed prevented me from going where I wanted to be.
4X4- unless you live where there is mud or snow many days each year, it's rare that you will need to use four wheel drive. I had it on one truck and only needed it once when towing and that was to get up a hill in a campground, I could have chosen a different campsite if I only had two wheel drive.
Hope this helps...good luck with your new rig.