Both islands are excellent, so it is not an option with a clear answer or a definitive recommendation.
One of the most important factors in your decision will be the climate and the time of year you visit.
I enter the variations from month to month below, but in general the weather on Phuket is better from November to January.the weather on Koh Samui is better from May to September.the weather is very good on both places from February to April.the weather is awful on both islands in October, when a visit to Koh Samet or Hua Hin would be a better choice.
If you only plan to visit one island or have a week or less, I recommend Koh Samui, it has a more Thai feel, handles crowds better (busy without feeling overwhelmed or incredibly touristy), and always has better food.
If you have more than 10 days to spend in southern Thailand and are open to the idea of seeing another island, divide your time between Koh Samui and Phuket. There are more than 6 direct flights per day between Phuket and Koh Samui and the flight lasts approximately 55 minutes.
Phuket VS Koh Samui
Phuket
Dense tourist atmosphere
10-12 good beaches
Good December weather to June
62 direct flights from Bangkok per day
1h 20m flight time from Bangkok
The airport is 60 minutes away by car of the best beaches
Many activities and day trips
Some difficulty in finding good Thai food
Many large spas
Very familiar
Few restaurants or bars on the beach
Lots of nightlife
Very safe removing some bad areas in Patong
Koh Samui
More relaxed tourist atmosphere.
3 or 4 big beaches
Good weather from January to August
24 direct flights from Bangkok per day
1h and 10m flight from Bangkok
The airport is 10 minutes drive from the best beaches
Few excursions and activities
Good Thai food is easy to find
Many small and medium sized resorts
Very familiar
Many restaurants and bars on the beach
Lots of nightlife
Very safe except for a few shanty towns in Lamai.
Climate by month of Phuket and Koh Samui
Climate in January
Koh Samui: The end of the wet season. The big storms have passed, but it can still rain every day on the island (especially at the beginning of the month). Even when it rains, the clouds quickly disappear and the sun returns.
Phuket: Excellent weather, sunny skies, little rain, calm seas.probably the least wet month in Phuket.
Recommended: Both good, Phuket a little better.
Weather in February
Koh Samui: Excellent weather from start to finish.blue skies, calm seas.
Phuket: Warm and dry, calm seas.best month for Phuket weather.
Recommended: Both, incredible.
Weather in March
Koh Samui: Perfect skies, perfect seas and perfect sun. The best month for Koh Samui weather.’, ‘
Phuket: Great weather with rising temperatures and humidity.
Recommended: Both, tie.
April weather
Koh Samui: Getting warmer but still blue skies and calm seas.
Phuket: Sunny weather with patches of rain.The heat and humidity can make some days uncomfortable.
Recommended: Both good, Koh Samui a little better.
Weather in May
Koh Samui Warm blue skies.a slight increase in the amount of rain.
Phuket: The beginning of the rainy season, but most days will still have plenty of sunshine.
Recommended: Koh Samui
Climate in June
Koh Samui Warm, humid, mostly blue skies.
Phuket: The climate is humid with many sunny breaks.The seas are getting rough and many beaches have red flags which discourage swimming.
Recommended: Koh Samui
July weather
Koh Samui: Warm, humid, mostly blue skies.
Phuket: Rainy season but sunny days are still common.
Recommended: Koh Samui
Climate in August
Koh Samui Warm, humid, mostly blue skies.
Phuket: Rainy season but sunny days are still common.
