Hello Saqib,The N91 has a metal casing and sliding front cover, which is used for controlling playback of music and other files. Sorry I cannot compare it with the N80 as I haven't seen one, but the N91 is easy to use and fits snugly in a pocket.
I have experienced the occasional crashing but nothing serious and it can be resolved by removing the battery and restarting the phone. I have noticed a significant improvement in performance with the latest version of Talks installed. the keypad is quite tactile with a groove on the 5 key. One of the best features is it has a standard headphone jack, perfect for plugging either headphones or portable speakers. One thing to bear in mind, the charger has a smaller pin diameter than the standard ones. Having said that, charging time is quite fast, around two to three hours with the phone switched on.
I hope the above is helpful. Best wishes, Clive Pallett