You might want to take a look at the Four Points too. I've stayed at it and the Westin, and I would choose it over the Westin. It's at Soi 15 and the Westin is at 19 if I remember correctly so they are real close to each other. In fact, after staying at the Four Points the last two times I don't think I'd stay at the Westin again.

If T21 and BTS are important to you then its hard to beat the Westin though.