I stayed at both hotels last October and I found the St. Regis to be the better hotel, but the Westin is in a better location if you're there for any reason other than business in Pudong.

I preferred the rooms at the St. Regis. The club lounge was great (better than the Westin) -- I think everyone has access to it, but Platinums definitely do. The butler brought fresh newspapers, fruit, cookies, and beverages each day and throughout the day (although I must say that we did have to put the "Do Not Disturb" on at one point so that the butler didn't keep ringing the bell with more amenities). After a day, the hotel staff greeted us by name as we entered. Also a great bathroom with a huge tub. We had no difficulty in communicating with the hotel staff as their English was quite good. There is some sightseeing within walking distance (Jin Mao Tower, the Pearl of the Orient Tower), but most things are on the other side of the river.

The Westin, however, was in the better location for most sightseeing and nightlife. Many more things are within walking distance. The Westin is a nice hotel, but it is much busier. The lounge is definitely not as nice (platinums have access). There is, however, a nice cafe on the ground floor where you can pick up coffee and pastries. Language issues were present -- we found more than one person who worked there who did not understand what we were saying. There is no butler and there are no goodies that I recall being delivered. The bathroom was nice, but it didn't have the deep tub as I recall.

I'm not sure what I would do next time. If I wanted to relax or had work in Pudong, I'd go to the St. Regis. If I wanted to enjoy nightlife on the Bund, I'd say the Westin. One caveat -- if you're not a walker and planning on taking a taxi everywhere anyway, I'd go with the St. Regis. Or, if you have enough time, maybe split your stay between the two.