We've only stayed at the Westin, and always on points. For just an extra 1000 pts per night (before the reval), you could upgrade to a jr suite, which is really a very nice large room with a fireplace and large sitting area. The last time we were there, they had changed the Heavenly Beds to a new type of Heavenly Bed, which we felt was not nearly as comfortable.
Now that the Westin has been reclassified to a 5 while the Sheraton is still a 4, we will definitely try out the Sheraton.
Also, every time at the Westin we were sent over to the Sheraton for breakfast anyway.