People have varying reception of Lean and Six Sigma. Some say Lean is better, others say Six Sigma is, and some others say that a combination of both is just perfect!
I say either or both is useful because it all depends on your processes, and in your process improvement goals.
In any case, PMcrunch has a good discussion going on Lean versus Six Sigma. The discussion touches on the what and the how, plus links to various resources that take a look at both methodologies from different angles.
Read the discussion HERE.
Six Sigma : Your Resource for Strategic Management. Meikah Delid. Eversun Software 2006.