The primary difference appears to be the urethane bladder in the fat cat versus the vinyl bladder in the fish cat. I am 5'11, 180, therefore a larger tube isn't a requirement. The fat cat is bigger by 10" and 2 pounds lighter (I don't backpack anymore). The cost differential appears to be about $130. Is the higher price of the fat cat "worth it"? Opinions and commentary appreciated. Enjoy in good health.