I've stayed at one or the other hotel for the past 7 months and would highly recommend the SLS hotel over the W in Westwood. Platinum members get free breakfast at the SLS, while at the W there is only a $12.50 / day credit available for use at the cafe. The breakfast at SLS is pretty good (no eggs or other hot items, though). The service at the SLS is better and more consistent than that at the W - I've had experiences where the W has forgotten to bring up my breakfast, phones in the rooms were broken, water pressure in the showers too strong, loud A/C in the bedroom, etc. The SLS hasn't had any of these issues.

The W does have free Bliss products, and their rooms are bigger (as a Platinum you would most likely get upgraded to a suite - about 80% of the W is a suite setup).

Good luck!