From what I have heard the dash is not much louder than the airbus and also there is pretty much no difference in the duration of the flight between the two-about 5 mins at most as the dash is not a lot slower than a jet on shorter flights and accelerates quicker.
Is it not more to do with it's ability to move more efficiently(quickly) between flight levels and therefore congested airspace? Hence the reason it is only quicker on shorter flights?