I've stayed at both Warsaw Starwoods and it depends upon where you want to be. The Sheraton is a bit older, more central (nearer shoppes, Old Town etc). It does have a very nice club with a good assortment of food for both breakfast & dinner. As a Platinum, I was given a large room-- but not a suite.
The Westin, on the other hand is near the old 'Jewish Quarter' (which has been rebuilt in the Soviet / block/ apartment style)...but there are a couple of good restaurants nearby and it is a quick cab ride or nice walk to the main part of town. I was there just after it opened and the while a Club was promised, it wasn't open yet. The hotel was very empty and yet no suite was offered. The Westin is very 'modern/W'-ish in style with a large open atrium lobby, while the Sheraton is more contempory.