Personally, for a foreign visitor, I'd suggest Cheung Chau over Lamma. Lamma is little quiet place, but Cheung Chau is more "local", more vibrant, with actual fishing boats still in the harbour.
But having said that, you're already going to Tai O on Monday. Why not visit some of the walled villages and heritage trails in the New Territories? Why not Stanley on the south side of HK Island? Why not Wong Tai Sin Temple and Chi Lin Nunnery in Kowloon?