Without a doubt the Westin. It also has an underground garage vs. outdoor parking at the Sheraton. I don't think I've stayed at the Sheraton once since the Westin opened.
The nicest rooms at the Westin are either the fireplace suites that
LIH Prem referred to (those also have a whirlpool tub and a shower for two, at least the one I stayed in did) or the "tower suites" which are at the corners of the hotel and have a huge outdoor patio. They're quite a hike to get to from the elevator, but well worth the walk, I think.