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...