Hi
Koh Tao is OK, but very touristy and overrun with dive operations.
When I dive Thailand, I head for the west coast as you suggest and it is just as easy to get to from Bangkok. Take the rain south or take a night bus.
I have dove several times with Sea Dragon out of Khoa Lak - http://www.seadragondivecenter.com/
Khoa Lak is north of Phuket
E-mail me if you need more detailed info - ed_arata@hotmail.com
If you like wreck diving - there is some in Thailand - but my favorite location are in the Philippines.
Ed