Tough call. Williams may be the better player, but Rondo is locked up at a very reasonable price for the next 4 seasons.
We know what Rondo brings to this team, which at times is jaw-dropping good or head-scratching bad. Williams I believe is more of a leader by example. Rondo can talk the talk from what I've read, but until he can really walk the walk, I question his ability to be the undoubted best player on a championship team.
I still believe the only PG I would deal Rondo for if the opportunity presented itself is CP3. Besides the overall talent, I think he is a guy in that circle and can actually convince D12 to come here, which is a plus (and late in games, D12 and CP3 is much better than with Rondo because both he and Dwight are horrible FT-shooters).
Rondo doesn't have the personality to convince players to sign in Boston, IMO.