more mob variety in un'goro but better rugged rate in winterspring yeti cave.
however, i will suggest another place...burning steppes. you have scorpids and dragonkin there. besides thick and rugged leather, you'll also get scorpid scales and dragon scales (i think the white worn ones, which are still useable).
dragonkin in swamp of sorrows may also be good to kill, depending on your class.
if you want to corner the turtle scale market, then turtles in hinterlands or tanaris, all the while getting other leather also.
keep in mind that depending on your server, something like heavy leather may have more demand than thick or rugged. you can kill heavy-leather mobs faster, netting you more over time. heavy leather on my server is actually not much less than thick or rugged.