I think it also depends on what you are looking for apart from diving, Koh Lanta is a lovely place very relaxed and peaceful, and I have also heard good things about Lanta Diver. I would avoid Phi Phi if I were you, as many dive centre's are looking for Divemaster trainee's(DMT's) to work also, and I have even seen with my own eye's DMT's being asked to take discover scuba divers out.
I have a centre here on Phuket and would be willing to help you if you decided to come here to do it.
What I would say is to choose the instructor who is going to do your course, not the dive centre, make sure you find out who your main instructor will be and how much experience he/she has, how many dives, how many certifications, how many DM certifications etc etc, as I have seen newly certified instructors teaching DM courses before and it is really not what you want.
I hope this helps you and feel free to email me if you have any more questions or queries.