I can’t say much about Koh Tao as I was only there for two half days and one night. What I do know is that it’s renowned for its cheap diving and so Orna opted for two days under water. However due to the price, the diving spots are more crowded there than in Koh Phi Phi and the waters more polluted from boats. Apparently the higher price is worth it in Koh Phi Phi for less crowding and better sights (Orna’s opinion).
My time on Koh Tao was spent roaming through pedestrian streets, dinner and sunset by the beach, another beach fire show, and a dip in the sea. The fire show was headed by a young teen who was amazing. The dip in the water ended in a downpour thus getting clothes and bags soaked and needing to borrow a towel and get changed in the tourist information centre across the road from the bungalow we had checked out of earlier in the day.
The island was still in low season when we were there but seemed like it could have a cool vibe in high season and if we had more time to explore.