There's no lounge at the SGS. The jazz bar there is FAR superior to the entertainment at the ROS.
The jazz bar is the lounge. If you stay in a Grande Deluxe room, you can enjoy free drinks from 6-8pm in the Library (jazz bar), Sala (poolside) or Versu (nightclub on the ground floor). Jazz Bar is great!
I prefer the SGS for its easy skytrain access (skybridge connected to Asoke BTS). The big, brown river doesn't hold that much of an attraction for me, but I guess if I'd never been there or hadn't experienced the river, I'd give the ROS a try.