It sounded like you're trying to depart from the airport in Vienna, in which case there's no point in going to Erdberg as all buses from there going to Hungary stop in front of the airport terminal as well. If you're leaving from the city then obviously take the bus from Erdberg (Eurolines.hu) or Praterstern (Orangeways.com).

If you're taking the train it makes a bit more sense to get off at the first stop in Budapest-Kelenföld, though its not as nice as Keleti station, you get in 15 minutes earlier. The cab (30 min) or public transportation (60 min) will cost the same and be just as fast. Public transporation: 49 Tram to M2 Metro to 200E bus, http://goo.gl/maps/9W4Nl.