Agree with most of the sentiments above.
SI's location can be a pain due to few attractions within walking distance. Getting in and out of the hotel is also a tedious as the road setup requires a multitude of U turns if going to/from Bukit Bintang. If you're prepared to take the monorail, you've got easy access to almost everything, so I'd highly recommend that. Hotel is looking decidedly tired (rooms and Club especially) but the service is superb, especially from the front desk and concierge.
Westin's location is a plus as has been well documented above, and the new Club is one of my favourites in Asia after the refurbishment. Rooms are also a bit tired (threadbare carpets for example) but this will be resolved next year when the rooms are refurbished to make it the clear choice in KL.