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Wait until you get to Koh Tao, so you can pick an op you like, it won't be any cheaper if you book ahead.
I would do it this way. You may also see what exactly is included in your package (books, confined water session in pool or in open water...) and decide whether you like the dive center and it's employees or not. Plenty to choose from once on the island.
Big Blue is also a nice dive center you might consider. If you go there ask for Rich. I've mentioned him on another thread, so why not here.
I'm sure on Koh Tao you can get the best deal for your course by just walking around and checking different DC's. Do not just go for "cheap-cheaper-cheapest" though because in the end you will only get what you pay for....