I stay at both frequently. I prefer the Westin over the W. You stand a much better chance of getting an upgrade. The only problem is that a bunch of rooms in the Westin smell like cat urine. They have a serious problem with dampness caused by airconditioners that leave the carpets wet. You get one free bottle of water.
The problem with the W is that you hardly ever get a suite. The room that my colleagues and I get upgraded to isnt even a full suite. It just has a larger room area and sits in the intersection of the two corridors. Of my ten stays I have only been upgraded once. They also give you a free water, a box of chocolates(sometimes expired) and magazines like wired.
My favourite is the Sheraton. Its in a remote location attached to Symphony Towers. They treat plats the best, two bottles of water, two coupons for drinks per day, breakfast buffet coupon and free internet/local phone calls.