I wish I could come with you.
My 2 pence, having toured Thailand last year. Go north. If you can spare the time there is a loop from Chaing Mai, up further north to Pau, and anticlockwise back to Chaing Mai. About 5 days riding. Hilly, beautiful.
I found the closer to Bangkok I got, I really didn't like the roads/traffic. Riding down to the south was pretty cool, but nothing on the beauty of the north. You can check out some photos if you scroll back enough on my instagram - Bencost