The two wireless headsets look identical and operate in the same way. However, they operate on different frequencies. The Plantronics CS50 operates at 900 MHz and the Plantronics CS55 operates at 1.9 GHz (DECT 6.0).  Plantronics CS55 is specifically designed not to interfere with other wireless devices.

Other differences are the talk and standby times.  The Plantronics CS50 battery provides up to 8 hours of talk time and 40 hours of standby time, while the Plantronics CS55 battery provides up to 10 hours of talk time and 50 hours of standby time.