The headset Discovery 665 has:
AudioIQ™ ; intelligent audio processing (exclusive Plantronics).
DSP (Digital Signal Processing).
3.5 hours of talk time and 80 hours of standby.
weights only 9 grams.
Case with led - call indicator.
The headset Discovery 650 has:
Multi point technology that allows the connections of more than one Bluetooth device simultaneously
5 hours of talk time and 150 hours of standby, with the AAA battery charging pocket
weights only 9 grams
Charging pocket with vibration alarm