Do you have status with SPG? what level? Plaza Athenee has a lounge for platinum members, which is a huge perk.

And what do you plan to do and where will you spend most of your time? PA is closer to sukhumvit road, but the choices within easy walking distance are so-so (in my opinion). Le Meridien is closer to temples, the river and Patpong.

There's actually tons of info about these places in the SPG forum, mostly from the perspective of platinum members.