I have had both. I love the Rhino spray on system. Takes a huge beating and still protects the bed. No matter what I carry, it will stay in place too, no sliding around like a bed liner. Plus I can fill my carry around gas tanks in the truck bed without worry of static electricity from a bed liner.
Saw a Line X liner in a truck just a while back. It appeared to me to be better than any I have seen . Had a nice, even appearance and was very nonskid. Looked like it was about a 1/4 inch thick so it should be very durable.
Biggest thing i see with a spray on liner is how the bed is prepped before it is applied. I've seen several where the liner is peeling and there is shiny paint underneath. Ask your installer if you can save a few $$ by prepping it yourself, thats the most time consuming of the process.