In my experience, all engines both 2 and 4 stroke are extremely loud. It is the mufflers that make them quiet, and it is the type and style and construction of those mufflers that should be compared to achieve a quiet running bike.
I agree that we need quiet bikes. Almost so quiet to be dangerous on the sidewalk, and any engine can be make quiet if muffled correctly.
Another idea, some styles of pipe/mufflers, especially for 2 strokes can improve HP, but I would prefer a 4 stroke with a large enough exhaust pipe and muffler to keep it quiet and not effect (lower) engine power. Easy to do, it will probably take up a little more room though.
Hopefully I am not just stating the obvious, but I think that if we really want quiet, we need to focus on the muffler, not the engine.