I had a nice stay at the Hilton Vienna am Stadtpark. It is a big conference hotel. I did not get an upgrade as a gold, but I was staying on a very reasonable points + money stay. Nice breakfast in the lobby restaurant. Busy, but a nice buffet.
I thought the location was the best part. Right across from a large transit center/mall. Literally one minute across a pedestrian street from the U Bahn and airport express. It was a couple of blocks walking distance to the innerstadt.
I bought a 3 day transit ticket and took the U Bahn, S Bahn, and Tram system everywhere. It was a beautiful fall Sunday, so I followed the locals to Schoenbrunn, which was a beautiful place to spend a sunny day.
I think the Hilton Plaza is a lot nicer, less business. But it was out of my price range that weekend.