The truth is they are all the same devices, the n85 has better battery life, bigger and nicer screen(oled) excellent music quality its fast both are fp2 also has 30 hrs of music playback, usb charging and the extra band for international 3g.
n85
Power Management
* BL-5K 1200 mAh Li-Ion battery
* Talk time (maximum):
o GSM 6.9 h
o WCDMA 4.5 h
o VoIP 6 h
* Standby time (maximum):
o GSM 363 h
o WCDMA 363 h
o WLAN 172 h
* Browsing time with packet data (maximum): 5 h 42 min
* Video playback time (maximum): 7 h
* Video recording time (maximum): 2 h 54 min
* Video call time (maximum): 2 h 42 min
* Music playback time (maximum): 30 h
* Gaming time (maximum): 7 h
n79
Power Management
* BL-6F 1200 mAh Li-Ion standard battery
* Talk time (maximum): - GSM up to 5.5 hours - WCDMA up to 3.5 hours
* Standby time (maximum): - GSM up to 15.5 days - WCDMA up to 16.5 days - WLAN up to 7 days
* Browsing time with packet data (maximum): Up to 5 hours 54 minutes
* Video playback time (maximum): Up to 4 hours 24 minutes
* Video recording time (maximum): Up to 3 hours 30 minutes
* Music playback time (maximum): Up to 30 hours