Phone options in Mercedes are quite complicated. As far as I understand you have 3 general options in older cars (before NTG 4?) with phone option: get a UHI cradle for your phone, use the Handsfree adapter or use the SAP adapter. I guess it will be more and more difficult to find cradles for recent phones like Galaxy S5 or Xperia Z3c, so you have to choose between Handsfree and rSAP. Samsung Galaxys usually support rSAP and all Android phones support Handsfree. The Xperia Z3c needs my app for rSAP.
Newer Mercedes (since NTG 4?) usually support Handsfree without any additional device. If you like you can still use a cradle if your car has this option.
Discussions about rSAP and general Bluetooth compatibility of the Mercedes-Benz Bluetooth SAP telephone module