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Both if you have 5+ days. If you have to choose: Koh Phi Phi for the postcard beaches, the island atmosphere, Maya Bay and snorkeling; Krabi (Ao Nang or Railay) for more choice of hotels, climbing, mangrove kayaking, more budget/comfort. Both are 1.5 hours by ferry from each other.
TL;DR
Combine both if you have 5 days or more. Otherwise, answer these 3 questions:
Are you looking for the "paradise" effect (Maya Bay, turquoise water)? → Koh Phi Phi
Do you want more choice of hotels + land activities + facilities? → Krabi
Are you traveling with children < 8 years old or have a medical need? → Krabi (Ao Nang)
Koh Phi Phi — the mythical island
Advantages
Maya Bay, Pileh Lagoon, Bamboo Island — Thailand's most photographed beaches are here
Island atmosphere — no cars, only longtail, night market
Snorkeling and diving — crystal clear waters, reef accessible from the beach
Tonsai nightlife — beach bars, fire shows, backpacker/couple atmosphere
Boundaries
Boat access only — ferry 2 hours from Phuket or Krabi
Few roads (especially pedestrian paths)
Basic medical infrastructure — no hospitals, just clinics
Expensive hotels of equal quality (island logistics)
Noisy Tonsai — if you sleep badly next to bars, avoid
Maya Bay strictly regulated (no anchorage, entry fee, opening times)
For whom
Couples on romantic weekend/honeymoon
Solo or duo backpackers
Divers
Travelers who want 1 wow instagram per day
Krabi — the smart continent
Advantages
Railay — peninsula accessible by boat, climbing cliffs, sublime beach
Ao Nang — convenient base, lots of hotels, restaurants, agencies
Hong Islands, 4 Islands Tour — magnificent day trips
Kayak mangroves Ao Thalane — nature option excluding mass tourism
Tiger Cave Temple + caves — varied land activities
Krabi Airport — direct flights from Bangkok, no need to transit through Phuket