There is no such thing as nightlife in Baden :-) .. Baden is famous for its nature and most people go there for a regimen, relax and so on.
Vienna is pretty cool ... there a lot of places to go. Just check out the city ("Stephansplatz") and walk around. Some known places are the U4 (
http://www.u-4.at ), Flex (
http://www.flex.at ), Fledermaus (
http://www.fledermaus.at ), Chelsea (
http://www.chelsea.co.at ), Kju (
http://www.kju-bar.at ) and many many others.
It all depends on what you are looking for. If you are more into alternative and underground, "Flex" is the way to go. The "U4" and "Fledermaus" have a different program every day, so it can be rock, r&b, disco,...
Chelsea is more on the alternative/rock side and Kju at disco, r&b,..