Charlottenburg (or Charlottengrad, as the locals nickname it due to always being a Russian-Jewish quarter) has many cafés, bars and well established night life...3 of the top ten night life spots are there like A-Train, etc. However, it is somewhat the yuppie chic neighborhood of Berlin...young professionals without children (opposite of Prenzlauerberg) and somewhat expensive.
Mitte is basically now only for tourist and rather boring for the locals.
P'berg, like ive said is basically young families, watch out for all the prams and strollers, as well as the other half (dead beat students refusing to get a job or self procclaimed artist, creating their next crayon masterpiece.)
Friedrichshain (or FREEKrichshain) is somewhat a deplicated, life-student, unemployed pseudo-intellect area...basically a ghetto...there are the one or two odd places, but trust me, if you dont know they exist, you wont miss them. Also a lot of right-wing extremist in several pockets of F'shain
Schöneberg &
Kreuzberg seem to be the most happening places in Berlin - great restaurants, shopping, parks...goes all hours of the night more likely with many options.