I haven't stayed at either property, but I stayed next door at the Melia Bali, and walked through the grounds and public areas of both the Westin and the Sheraton.

This is one case where I think most people will prefer the Sheraton to the Westin. Both are fine properties, and both touch the same beach, but the Sheraton seems more like a total resort while the Westin looks and feels a bit more like a business or convention hotel--though it's located in one of the world's prime tropical resort areas. I felt that I wouldn't be unhappy at the Westin, but that the Sheraton "showed off" a bit more. Or course, I didn't see rooms in either one. Both seemed very well maintained.

Since the Sheraton costs a bit more, the equation is pretty even.