Danube
As other posters note there is no lounge at the Hilton Danube. The rooms are large and as a Diamond I was placed over the river in a nice room with a small sitting area by Bay windows. The breakfast buffet is overall very good....thought that the service was a bit suspect. The front desk was very pleasant and helpful.
There is a tram about 5 minutes from the hotel that takes you to Vienna. The only two negatives of this hotel is that there was only a very small gym and really no restaurants within walking distance.
If you get a cheap rate then I'd say here, otherwise I'd probably try one of the Hilton's in the city.
As a note, If your flying in....when you get a taxi at the taxi desk they ask if you want a round trip taxi...if your taking a taxi back I'd definately recommend it. There was an 11 euro surcharge for the taxi going out to the airport on top of a 27 euro ride. Think that if I'd had gotten round trip it was 25 euros each way.