I've had both, and each has their advantages of disadvantages. Currently have a spray in liner, but to add some extra durability to it, I added a rubber mat. At TSC, they sell vulcanized rubber horse trailer mats that measure 5'x7'x½". I cut it down to fit between my wheel wells, and snugly against my tail gate. I can toss cement blocks into the box without worrying about denting the floor, or sliding around. You could put one in a slide in liner if you wanted to keep things from moving around. They come on sale regularly. In fact, I just checked, and they are on sale this week in the flyer for $52.
http://www.tscstores.com/5-X-7-X-12-...x#.VPXlBfnF_7E