Simply, size. WestRim 9 is designed as a day hammock. 9 feet long and approximately 58' wide. Not to say you couldn't camp in one if that was the size you were looking for. The WestRim 11 is, you guessed it, 11 feet long and approximately 74" wide and is available with an optional ridgeline. Neither has an attached bug net but can be used with our cocoon style bugnet.