I just came back from Berlin (awesome couchdb conf @ co.up co-working space) and must say that the energy in a place like Kreuzberg is pretty intense.
The area around Rosenthaler Platz and Rosa-Luxemburg-Platz in Mitte also seemed kind of interesting but more akin to what I'm used to here in Vienna (neat and tidy).
I only had five days to experience the city but I do have thought about moving there already!
But yeah living in Vienna is so damn comfortable that it really is a difficult decision to go anywhere else.
We have a lot of co-working spaces too and I often meet local devs at coffee shops by coincidence (another strength of a rather small place). So everything being so nice is both a good and a bad thing I guess.
Vienna #1 at quality of life: http://en.wikipedia.org/wiki/Mercer_Quality_of_Living_Survey
Bottom line for me though - the youthful vibes in Berlin are a very special thing and probably worth more than top comfort and beauty.