What about the Radisson Blu Falconer on Frederiksberg?
It seems cheaper than the other two ( for a random date in beginning of December).
It's not in the tourist center of town as such but at least it is in a nice area with shops and such. It is a much livelier area than the Radisson Scandinavia. There is a very nice street ( Gl. Kongevej ) that will lead you straight into town either by walking or by bus.
Very close to a metro stop and also close to busstops with several busses that'll take you into town.
Transport to Ballerup is also easy from there - you won't need to backtrack to the main station. You can get the metro at Frederiksberg metro station, change at Vanløse Station to the train to Ballerup Station. The whole trip will only take 20-25 minutes.