Compared to the cheaper 1600 series the 9600 handsets have larger display and everything tends to be on screen and digital as opposed to flashing lights and names printed on the side of the console. They also offer Gigabit Ethernet standard whereas the 1600 series need an external adaptor. Outside that, the feature sets are basically the same offer the same overall functionality so it often comes down to aesthetic, user preference and price. You can mix and match from all current Avaya handsets (IP and Digital).