Just wanted to add my 2 cents after staying at both ROS and SGS...
Both offered excellent Plat recognition and had their advantages.
ROS is great for first-timers and those wanting to do the touristy things. Having both the hotels boat to the SkyTrain and the public boat next door to all the river stops is a great way to avoid traffic and see the city. Lounge is top notch with both breakfast and evening cocktails. There is not a whole lot in terms of restaurants within walking distance, but all the basic street food, 7-11, etc. Bottled water & fruit were endlessly refreshed.
SGS is a bit more business-y and upscale. Having the skytrain outside is a great way to get to all the malls and major commercial areas. There is also a lot to eat, shop and see withing walking distance. While no lounge, the offer of cocktails in 3 different venues + breakfast was great. Butler service brought up cappuccino as advertised and the packet with internet code was appreciated. Check-in desk was mobbed upon my arrival, but several agents took us out of line, asked us to have a seat in the lobby and checked us in while sitting down. Check in agent also escorted us to the room and reviewed all the amenities. The suite had decor and amenities on par with a St. Regis, Four Seasons or Ritz. No Pantene products in the bathroom here, it was Ferragamo!
Tip: at either hotel, don't forget to ask the doorman for the card with the hotel address in both English and Thai for easy taxi request back.
Starwood really shines in Thailand ^