I've stayed at both and give the Sheraton Gateway an edge for these reasons: somewhat quieter (the Westin is at the end/start of the LAX south complex runways [depending on how the runways are configured]; Gateway is across Century Blvd.); full bkfst is given to Club level guests whenever the lounge is closed (Westin has NO club level) and I can often find free street parking behind the Sheraton, not so the Westin.