Not sure which one you have but the B&W gooseneck is an underbed mount that you can use for the gooseneck and B&W makes what they call the companion 5th wheel hitch that mounts in the same hole to the gooseneck mount. So you can have both. The companion is removable so that you can use the gooseneck (two pins to remove the head, one bolt to remove the companion base takes about ten minutes to switch from one to the other). There are surely other brands that do the same.
http://www.turnoverball.com/products/single-point-attachment/companion-5th-wheel-hitch Again it will depend on what you have as to whether it can be done or not. I love mine for three reasons first I can tow both types of trailers, second the companion is a great 5th wheel hitch and third if not towing either I can have a completely flat bed with no rails for the other work the truck does.