I'd say the Line-X is about the best there is-I know a lot of people who've gone with Rhino and have been very disappointed with large sections bubbling off, then having difficulty with warranty repairs on them. Only know of one guy with a Line-X repair, that was fixed no questions asked, and not a problem since-turned out he had some rust issues under the paint in that location too, so it really wasn't the liners problem.
As for Herculiner, I did my entire trailer in it, which I pull mostly up gravel roads and such. I did two coats on the thing-concerned about the gravel chipping it off, and couldn't be happier with it's performance. I don't know how many thousand miles are on it now, and I've replaced the tires on the trailer-the Herculiner is still in great shape. Only spot missing is where I had to grind it off to weld the tailgate tab back on.
I think with any of the above suggestions for bedliners, the actual preparation of the area to be coated is what is most important. Follow the directions exactly as they state for initial prep work, I think you'll find the final job will come out a lot better, and last a lot longer.
Best of Luck,
Mike