I don't think there's even a question, Vienna. It's a very historic city, with a much larger and more active [observant] Jewish community. Frankfurt, although a historic city as well, seems to serve more as a thriving modern metropolis. It doesn't have nearly as much historic importance as Vienna.
#mytwocents
in a 12 hr stop, he might prefer to go to worms, michelstadt and the beis hakvaros of fra rather than getting an extremely limited exposure of vienna.
its a matter of preferences, so yes, there is a question