Both the Rhino and Line-x liners are made of the same type of urethane just slightly different to make them their own. The type of urethane they are can not be made UV stable, so they are both prone to fading. The amount of exposure to the sun will determine how fast. No matter what liner you end up with, the main thing I would check is the workmanship of the person applying it. I had been in that business for some time and have seen good and bad from both of them. It usually isn't the product, but rather a poor prep job or application of the product.
After having worked for one of the above companies and a friend working for the other, I went with a Superliner for my truck. It is a different type of urethane that mixes with automotive tints to give it's color, including black. Because of this, it is UV stable and I was able to color match my truck including adding the metallics to the color. It doesn't go on as thick as the others, but it doesn't need to since it's tear strength and elasticity is greater than the others. The only draw back is if it were a real work truck where things were being thrown into the bed, it could dent the bed easier. If it did dent I would rather have the softer material so it would stretch with the bed, rather than seperating from it. Just my opinion... Andrew