First - your dealer is wrong.
You definitely want to get the 5th wheel prep kit from the factory. Doing that means Ford drills the holes in the bed, etc. They will also provide a set of plastic covers to fit in the holes when not using a hitch. I believe the electric plug-in on the side of the bed is also part of the package.
You then buy the 'picture frame adaptor' that will allow you to use many (all ?) of the commonly available fiver hitches. I did all this when I ordered my '13 dually and then added a Hensley air-ride hitch (bought the adaptor from Hensley).
There is really no reason not to get the factory package - unless you want to drill all those holes and add the reinforcing yourself.