With only two nights in Bangkok I'd really stress the location of the hotel and would skip ROS. Have dinner or drinks at one of the riverside hotels instead and choose a location with convenient BTS access - SGS, WGS or StR. Plaza Athenée to Ploenchit BTS isn't that bad, but for a first timer on a tight schedule, time is of the essence

I'd then decide on which hotel based on what's most important to you - style, price per night or facilities (e.g. nice pool).

As it gets dark in Bangkok at around 7pm I wonder if you'll really enjoy the overnight train - you won't see anything beside the train.