Sail Rock is closer to Koh Samui than Koa Tao however, Sail Rock is a small rock pinnacle hardly recognised on the map and therefore there are, allegedly, strong currents. As such, not for the novice diver. I would expect lunch to be a little bumpier also.
Kao Toa is very sheltered easy diving with the same critters a you would expect see in Egypt although the vis isn't as good as Egypt.
Sail Rock is closer and as a result cheaper than the long trip to Koa Tao albeit only by 10 - 15 quid !. Whale Sharks re said to been seen at both spots but how often have you heard that one ?? I plumped fro Kao Tao so Nick could do some snorkelling. If you plan on taking Megs, Sail Rock sounds like the currents could be challenging so careful consideration is needed..
I paid 3900 TB approx 78 quid for a days diving including breakfast, fruit, drinks and lunch. I had my own equipment. Without equipment this rises to 5500 TB - approx !!0 quid. Sail Rock will set you bak about 100 quid for the same days diving.
There are loads of dive centres around Chawang & Choeng Mon. You will get a good deal if you shop around yet the prices seam to be around 5000 TB - 5500 TB (inc equip).
Let me know if you need any more info ?