I stayed a couple of times at the HI Ploenchit and was very happy with the place. Got it for about 3,000THB on a F&F rate for the exec level which includes free internet, lounge access and breakfast downstairs. I dont know if that rate would hold for your time period but as a Plat you may be able to book a lower category and score and upgrade. I had only Gold status when staying there before.

If you are want to stay with PC then you may want to consider the HI Silom. I believe the hotel itself to be a btter property and from looking at some of the posts in the PC forum they are prettye good about upgrades. The location just isnt that great.

I dont think you can go wrong with most of the serviced apartment locations like Aspen, Centre Point Wireless Rd and the Siri Sathorn mentioned earlier is a nice place and not too much of a hike from the BTS especially during this time of year when the the weather is better. I have also stayed at the CentrePoint Saladaeng which is nice enough but a bit dated. It is just down the street from Siri Sathorn.