The cities which have ( or had) both Sheraton Grands and Westins that I am familiar
with are Taipei and Seoul.

I consider the Taipei Sheraton and Westin exactly the same in terms of service and
quality, and book my stays strictly with whichever is offering the lower price.

Until the Seoul Sheraton Grand left SPG Jan 1, I always considered the Westin a
slightly better hotel, but would stay at the Sheraton if the price savings was
greater than 15%.

As others have mentioned the SGS in Bkk is a Lixury collection, not a Sheraton,
so the comparison is not fair.