Just one more idea: if the Peninsula is running the suite life promotion during the time of your stay, a Grand Deluxe suite at the Peninsula at THB 11,000 per night is a no brainer. It`s still a good deal at the Virtuoso rate, or through FHR, even if Suite life is not running.
Sorry
Blumie, I had misread your post and thought that this was for a single two night stay, and therefore recommended the Peninsula and suite categories at the Four Seasons.
For one night stays, the Four Seasons is the best choice. Personally, I would book a Deluxe View Room, with the hope of ending up in a Premier room. I would try to get the Virtuoso or FSPP upgrade confirmed in advance. One thing I like about the Four Seasons is that you can pre-order a nice room service meal if you are arriving late at night.