All your options are good. It depends what you prefer. You are deciding between Vienna plus Salzburg or Berlin. Vienna is classical grandiose capital of a former great empire. Salzburg has a beautiful setting in the Alps with a small downtown and fortress on the hill overlooking it all. If you decide for V+S I would put 3 nights to Vienna and 1 night to Salzburg. Berlin is huge. Very exciting, full of stuff to do, many museums, very spread out, no real center, architectonically mishmash of old and new (more of new) and nobody can accuse it of being pretty. It's the most different from your other cities (Paris, Budapest, Prague - all eye candies), that's why I would choose Berlin instead of Vienna plus Salzburg.