I just returned from 10 days in Thailand. I would highly recommend it for a vacation locale. It was my second time in Thailand, but my first time actually vacationing there - the other time was on business. I could easily see splitting two weeks between Bangkok, Phuket and Chiang Mai.
I have heard wonderful things about Bali from friends who travel there frequently and my H and I plan to take a trip there the next time that we travel overseas. However, with no real experience, I can only truly recommend Thailand.
We stayed at the Millenium Hilton on BKK (on the river) although if your budget permits, the Peninsula was right next door, and it's one of the most affordable Peninsulas in the world. (Our hotel was paid for by my company - otherwise we would have splurged!) In Phuket we stayed at the Moevenpick resort in Karon Beach in a Plunge Pool penthouse Villa. I would highly recommend this resort.
Good luck with your decision!