I've found both of them average, although the top floor suite at the Sheraton, right next to the lounge, was much nicer than the suite I had on Wednesday at the Westin.
I had 2 Westin stays this week but the Sheraton has been a couple of months. I have 2 nights at each coming up this week, so I'll be able to comment with more recent data.
The Westin doesn't provide a newspaper in the morning, nearly a dealbreaker for me. Half of the room also are next to a heavily travelled, noisy road.
I'd say the Sheraton is average for the brand, the Westin below average for the brand.