Just to clarify what has been said,
Do not buy a 100 GPD membrane, and do not buy anything other than unit with a true dow filmtec membrane. There are other membranes the use the dow filmtec materail, but they are not DOW membranes and do not perform as well. Also, anything that is 100GPD or greater will not perform as well as the 75GPD with regards to rejection ratio (this determines how long your resin lasts).
Bean