If you are installing something like a B&W gooseneck hitch I would install the hitch first. That way the coating can seep down and cover the bare sheetmetal exposed when cutting the hole in the bed.
That said, I think they are all a huge waste of money. $69 will buy you a nice rubber mat right from Dodge that will keep things from sliding around. I have never understood the logic of spray liners, or liners period. The idea is supposedly to protect the bed. From what? For what you spend on the liner, you could repaint the bed before resale. Besides, they all wear out, wear thin and discolor over time.