"Extending" on n base stations is easier to set up and is supposed to be more efficent than WDS.
WDS is backward compatible with older non n base stations.
Except for the most recently released base stations while n and g base stations can be set to connect to each other, n base stations will be forced into the slower g mode whenever a g connection is active, meaning a WDS between n and g stations would keep you at g speeds all the time, which is why dual wireless networks became a popular configuration. Basically, you connect your slower network to your faster network through a wired connection.
It makes the most sense to use "extending" between your n base stations, connect a g base station to that through a wire and then connect your b base stations using WDS, assuming you need your network range expanded that much.
The latest batch of Apple routers support g and n connections independently allowing g connections without slowing down n connections.
