We had an amazing stay at the Westin 6 or 7 years ago- they upgraded us to a beautiful suite- surprised us with with wine, cheese & fruit and gave us a beautiful book on Rome. The concierges were top notch and they really made us feel special.
A year and a half ago we were a little underwhelmed by the cold service at the St Regis. Our room was fine ( we applied
SNAs) but we found the hotel uncharacteristicly stiff compared to other hotels in Italy. ( after that stay we had an amazing 5 nights at the St Regis Florence)
We are typically big fans of the St Regis brand- but I think we would choose the Westin on our next visit.