A few things for you to consider before you decide:
1: Time of year : The Gulf of Thailand (Koh Tao/Samui/Phangan) and the West Coast (Phuket/Krabi/Koh Lanta/Phi-Phi/Khao Lak) have their own "best" dive seasons. Doing your course(s) at the right place and at the right time will give you a better chance of doing your training and "fun" dives at a variety of dive sites.
Although the advice regarding the seasons makes sense, it almost doesn't apply to an IDC setting, since unfortunately, an IDC is completed almost only in a classroom and Confined water setting, with only two required Open Water dives.
If you want and can afford (time wise) to do a lot of fun diving, than the choice becomes a lot more important.
2: Time you need to finish your course(s): If you have only limited time it's better to do your courses with a well-established Dive Centre with a steady stream of students than with a hut on the beach with the occasional walk-in customer. You get your best DM training dives with real students and a lot about becoming a good DM and/or Instructor is about taking care of, supervising and teaching student divers.
Some shops with few student diver customers may use their DM'rs as "student diver" during your course(s).
Why do you bring in a Divemaster course when somebody enquires about an IDC?
This OP never mentioned a DM course.
You bring up a good point but a moot one for this discussion/enquiry.
3: Visa: I guess you will be arriving in Thailand on a Tourist Visa and this will give you 30 days in The Land of Smiles before you will have to either extend or renew it. Getting a new visa means a "border-run" or "visa-run" and you will have to leave the country for that. Travel time can vary greatly depending on where you are doing your course.
Good point. Most shops that offer IDC's will help their candidates with obtaining student visas. I do offer this service, providing a booking has been made before the start of the course(s).
4: Personal preferences: Do you want to stay just on a beach during your course or would you prefer to have the beach and a whole lot of other options as well?
Believe it or not but it does make a difference after a couple of weeks...
Makes a big difference!
5: With who: Succesfull IDC/CDC Dive Centres are not necessarily "factories" even if they seem to have so many students. A lot of personal attention is given to IDC candidates by most Course Directors and most IDC/CDC Dive Centres I know truly do offer good and professional courses. Remember that successful Dive Centres are mostly so successful because they do a proper job! I recommend you do a bit of Googling, contact a few IDC Centres and wait for their responses. If they reply to your messages promptly, are happy to answer all your questions, are polite and give you " a good feeling" than that may be the shop for you!
Excellent advice! Make sure that you're taught by a Course Dircector and by his IDC Staff! In general, Thai Course Directors are very experienced since they offer IDC's on an ongoing base.
