I believe it's an unresolved question whether Athenee has renovated all rooms. If mine (early March) was renovated, it was an extremely light remodel (and I really don't think it was).

These are both good choices. Both have nice pools, with the Athenee's being more spacious, the StR's has a view. The Sports Club view at StR is really nice (Grand Deluxe rooms). Location is similar. At StR there is direct connection to BTS, though you have to change at Siam for Sukhumvit line. StR has a more overall luxe feel and nicer lobby. You'll also have butler service at StR. Athenee has the lounge (and while I didn't try it, the breakfast at RainTree is reputed to be one of the best in Bangkok).

These hotels are also both well located for a morning drive to the airport if you wanted to stretch to two nights.