For me, definately leather. First there's the wonderful erotic smell of leather. Then, I like the skin-on-skin feeling combined with reaching down to stroke the leather covered side bolsters as I'm driving down the road. On long trips, I like to spread legs and let my balls slap the seat over expansion joints. Leather is also easier to clean up than cloth.
Ok, so which do you prefer to masturbate in, cloth or leather? Why?