I personally find the Eheim 1048 to be a little quieter than the L20 in side by side comparison. The L30 and Eheim1250 are nowhere near as quiet as either.
The L20 is super tiny though. If space is an issue go with it. The noise level isnt a huge difference; I dont even know if you could tell between them if they were in a case.
http://www.procooling.com/~phaestus/l20vs1048.jpg
Never touched a viaaqua so can't comment.