As always, it depends. Do you care for location? IC is definitely in a better location to the city center and easily walkable versus Indigo and voco. Indigo is a nice hotel (bit smaller, but smart rooms) and good access to public transport and a bit of a longer walk into city center. The Prater location of the voco may suit some and not others. Can't say anything about the hotel itself as I haven't been there.

Rates / $$$ and potential status benefits associated (or lack thereof) could also swing the decision from one to the other. YMMV.