I am at the Westin now. I come here every 2 weeks for 1-2 nights. I've tried the Sheraton a couple of times and have been unhappy each time. I am also a Platinum, and they tend to take care of me, but they are often sold out and cannot give me an upgrade. My problems at the Sheraton have included very noisy rooms and rude staff.
Note that they are owned by the same operator.