A stiff arm is a push and a clothesline is more of a hold.
You would stiff arm a guy away from you because pushing is legal but a clothesline would be more along the lines of a guy getting past you and you needing to put your arm out to stop linhim. By then it's too late and would be considered a hold.
What about a clothesline to the neck?A clothes line would basically be a helmut grab, too. I don't think you stop anyone in the NFL with a clothes line to the body.