Although they are all Starwood, my personal experience is that there is a big difference between Sheraton and Westin. My experiences at Westins have almost always been better than at Sheratons. This difference, in my opinion, existed long before the merger and unfortunately has persisted. Others who stay more frequently may have a different perspective, but my suggestion would be to try some Westins before you give up on Starwood.
My personal rating system puts Sheraton and Hilton together, both below Westin, Hyatt and Marriott, in terms of both service and accomodations.