Westin
I've stayed in both and the Westin is a nicer hotel, plus its better located to the downtown resturants and the highway. However the Sheraton is normally $30-50 less expensive, plus less expensive parking. The Club lounge in the Sheraton is also nice, with a quick nice easy breakfast.
So I guess you get what you pay for. Is a better bed and a better location worth $40 - $50 - only you can answer that.
Just avoid the Four Points in Monrovia. Its less than $100 a night but sooooo noisy. The rush hour begins at 4am and ends at 3am . The hotel is less than 30 yards from the highway and is positioned so all rooms have a highway view!