Singles will generally be a broader pattern, where as dual antennas are not tuned to a ground plane but instead to each other. They don't reach out the sides very well, but will reach forward and rearward better than a single. If you are pulling a trailer and trying to make it back around the trailer, this is an asset, which is why most OTR trucks are running duals. I ran a single on my 95 mounted on the the front fender half way down the hood and it worked very well, ran a 4 or 5 ' fiberglass Firestik 2. The roof is the best ground plane, the hood is the next best thing. The box makes a poor ground plane, so if you want to mount behind the cab either run a dual setup or make them tall enough to ground plane off the roof. However, the added interference of being positioned right behind the cab will negatively increase your SWR. Perhaps a better non-roof mounted option would be a set of duals off of the front fenders. Please note that you must use specific wire and setups that are different if you run a single or duals. You can't just T two antennas in and expect it to work properly. That is how radios are burned out.