Most boats from Koh Samui will bring you to the same Koh Tao dive sites. You're better off staying in Koh Tao to cut down on boat travel time (< 20 min for most sites, vs 1+ hr from Koh Samui).
Sail Rock & Chumphon Pinnacle are truly fascinating. They are both much further out, about 1-1.5 hour boat travel time, and you have to check with your shop when they're going there. Typically shops will organize a trip once or twice a week to these sites. Also loved the Sattakut wreck -- it's largely intact and a fascinating ship.
I liked diving with
Hydronauts. It's a smaller outfit so you're not crammed with a million other divers on the boat, and they were very accommodating with my schedule. They moved their boat departure times to 10am for me, because I didn't fancy waking up early on vacation
They were also the only shop I found that offered BPW/long-hose gear. Definitely recommend reaching out to them.