If you have an option of either Andaman Sea/Similans or Koh Tao, obviously goto Andaman Sea.
I did both. (Wanted to get a quantity of dives out of my system!)
From a recommendation on this board (a technical diver) they suggested Master Divers.
I would also recommend Master Divers as they are on the quiet side of the island, and, as a result there are less divers there. There were good diverse range of guides from all over the world.
I completed the EAN Enriched Air Nitrox course with them.
I also brought my own gear, but it did seem like their other gear was kept well. If you are unhappy with any equipment, they would be happy to change it.
Master Divers have regular 'island clean up' sessions. They also have an agreement you can 'sign' to pledge that you will ask to not have a straw in your drink, as they tend to end up on the beach and in the reefs.
With their location, they are only about a 150m walk from the main ferry terminal (turn right when exiting) - I also stayed down that side of the island.
In the evenings, it is much more quieter than if you were to turn left when exiting the ferry terminal.
The Coconut Monkey Cafe is quite nice - they use reusable bamboo straws.
If you have any other questions about getting to Koh Tao, please feel free to ask me. I tried to get there with minimal time on those ferries. I flew to Koh Samui with Bankok Air (they are quite a nice small airline - you will be impressed!) - then a ferry from Koh Samui to Koh Tao.
<Edit>
I forgot to mention about the ratios. I think they have a 1:5 ratio.
During my trip there, it was a 1:3 ratio (myself with two friends!) - and the other guide was a 1:2 (due to language) - After every dive, there was fresh fruit, biscuits, electrolytes, tea/coffee.
They have big boats with WC.