I had Rhino lining sprayed into my uncle's Dodge Dakota back in '92 or '93, then had my Ford Ranger's bed sprayed in '95. Both are still holding up well. My uncle's truck is used on a farm, and I use my truck to haul dirt bikes and the accompanying gear. There's been a slight lightening discoloration of the black finish in both trucks over the years, but so far no major problems. Have only had a few nicks.
Besides the durability, I really like the fact that Rhino linings are fairly thick (1/4-inch, if I remember correctly) and aren't that slick even when wet. Even after a muddy race or washing the bike, I don't have to worry about the loading ramp or tires slipping when I push the bike back up into the bed. Can't say the same about drop-in plastic liners that warp with age and are occasionally seen along the sides of freeways.
I recommend Rhino highly. Have no experience with Line-X. Had both trucks sprayed at Rhino's HQ in San Diego. Granted, it isn't cheap, but it looks like they'll last as long as the trucks. [^]