I'm a fan of the Rhino liner, primarily due to the fact that it tends to help keep things in place as cfabe said.
The Rhino is more of a rubber feeling coating, while the Line-X seems to be harder. My uncle had Line-X put into his truck, and the salesman showed him that taking a piece of metal with Line-X on it and rubbing it onto a piece coated with Rhino would tear up the Rhino liner. Given Line-X is harder, I'd belive that, so it might be a touch more durable. However it's been my experience that if I want anything to stay put on Line-X I need to tie it down. A good friend of mine has had Rhino in his truck since '98 or '99 and we've been very rough on it, and everything stays put when travelling around. In fact, it can sometimes be a challenge moving around in the back of the truck on your knees or with rubber soled shoes because the liner doesn't let you slip and slide around. Only one spot on his has been damaged, and that's due to a heavy tool box rubbing its corner in the same spot over probably a year's time. Shifting the tool box back solved that problem. Otherwise, no issues with it at all.