Try to stay at any of the hotels facing Danube. View is excellent. Highly recommend Marriott or Hilton (the ones that are on the river).

pick-pocket is an issue. There is no direct train/subway from airport (have to change using buis E200) and would not recommend doing that after dark for the first time visitor. You can join share-a-taxi to downtown for less than 10 euro.

Local transport is excellent but if you are pressed for time, hop-on-hop-off alternative is fine too.