Except for the Smartplug that comes with the RX65, to the Corded plug that comes with the RX55, which has nothing to do with it detection capabilities, the only difference is the Radar Receiver/Detector Type.
RX65 - Radar Receiver / Detector Type
Superheterodyne, GaAs FET VCO
RX55 - Radar Receiver / Detector Type
Just how much better can the GaAs FET VCO technology be to the VTO technology?