Nuno,
what sort of budget do you have? In most of those places you've listed, there are 5* to backpackers places.
Railay beach is a known party spot with DJs like Fatboy Slim and Sasha making appearances when they stay at the Rayavadee. Very picturesque place too, but you lose the beach in June due to high tide and the winds and rain picks up on the entire Andaman side. Staying on Krabi/Ao Nang is ok too. I prefer diving from Phuket myself because the boats are nicer.
PhiPhi also has a variety of dive ops and resorts for all budgets. If you want to dive Hin Daeng/Muang, move down south and stay at Ko Lanta or somewhere closer, the long ride from PhiPhi/Krabi/Phuket isn't worth it unless you do a liveaboard, especially during June. Weatherwise, June isn't the best time to visit the Andaman side of Thailand.
However, I'd rather visit a windy/rainy Andaman side than go to Ko Tao/Samui/Phangan for more than 3 days. Featureless islands with badly developed places and only 2 real dive sites to speak of. But that's just me... I never got into the scene at Ko Tao, and the 3000 divers which share the same sites is just chaos sometimes. Underwater, it's not as nice as the andaman side overall. But that's my opinion (plus I'm a Phuket resident part year
)
If you want far off adventure, instead of Sipadan, try Pulau Weh near Sumatra. It's an easy flight from Penang and the diving is better than Redang.
If you give a budget, I can recommend resorts for Thailand.