Go to Tao, spend some time walking around the island and checking out the different dive operations there. You'll get a far better appreciation of the strengths and weaknesses, plus an ideal of your personal 'match' with the instructors, by meeting with them face-to-face.
Gauge the overall professionalism of the dive centre. Talk to their current DM trainees. Have a look at the quality of their regular students, their rental dive equipment, their dive boat, their shop etc etc. Judge for yourself which centre produces the most high quality divers. Be wary of the 'sausage factory' dive centres. Some are good, but you won't ever be treated as an integral part of the team. You can normally spot a good or bad 'ethos' the moment you walk into a dive shop... trust your judgement and don't be swayed by cheaper prices.
Do some research here on Scubaboard to identify what constitutes a 'good' and a 'bad' diver. Apply that judgement to the dive shops you visit.
When you first arrive, get some cheap accomodation in Mae Haad. You get more for your money there. Rent a moped to get around. Spend a week visiting dive centres (and go diving with the good ones for a more in-depth evaluation). Also..talk to the dive staff around the island about available long-term accomodation. The cheapest deals are normally away from the main tourist areas and are a fraction of the cost of short-stay rental rooms and bungalows.
Shops to definitely check out:
Master Divers (Mae Haad). A small, highly professional shop. Ayesha Cantrell (manager) is here on Scubaboard.
Crystal Divers (Mae Haad). A large dive resort. Not as personal, but maintain good quality standards.
Seashell Divers (Sairee). A small dive shop, very friendly with a very motivated manager (Mike).
Big Blue (Sairee). A large resort, very active and a friendly bunch of people.
Ones I don't Like:
Scuba Junction (Sairee). I always found that the manager had an 'attitude' when it came to DM training.
Bans (Sairee). A very large resort. I found that they did too much partying. Over-large courses, consquently their students were bottom churning rototillers.