The second night complimentary at the Pen requires a minimum rate of THB 20,000++, or roughly USD $760 all in, not that much more than the FS rate you quoted. IMO, both are greatly overpriced for Bangkok. Is a suite important to you? I think that you'll find that the Pen, while a lovely hotel, is poorly located. FS is in a much better, more central, location. Also check out the new St. Regis, if that level of luxury is what you're after. The tourism business is slow in Thailand right now. Don't book online. Decide what you want, then call the hotel directly (not the central reservation number) and see if they will give you a better rate.