It's a topic that seems to come up quite regularly on the discussion boards: Whether to go to Samui or Phuket. The question usually revolves around the weather. The two islands do have distinctly different rainfall patterns. If you're planning your trip for January to April, there's not a significant difference, but from May to September Samui is quite a bit drier than Phuket. The situation then reverses itself from October to December. We would be remiss if we didn't point out that, if you want to travel around June to August, and want to be sure of a rain-free holiday, then you should consider Bali, which is having its dry season while Thailand is in the midst of its rainy season.
Aside from weather, there other considerations. Although you can get to, and stay, in either destination on the cheap, for the average traveler Samui is a bit more expensive. It's actually just basic economics: everything has to get to Samui by boat or airplane, adding extra costs to all the basic necessities of life. On the other hand, Phuket is connected to the mainland by road, making it cheaper to bring in all the goods needed to support a large tourist population.
We should also note that air travel is a bit cheaper to Phuket as well. Samui's privately run airport, owned by Bangkok Airways, maintains rather high landing costs compared to other Thai airports. Phuket is served by several airlines, including Thailand's national carrier, Thai Airways as well as low cost airlines such as Nok Air.
Aside from costs, both destinations have ample amounts of find white sandy beaches. We tend to think that the north end of Chaweng Beach on Samui is about the best beach there is, but that's a very subjective personal opinion.
Away from the beach, both destinations have access to excellent dive sites, and some spectacular scenery. As a larger island with a longer history of tourism, Phuket probably has a greater selection of other diversions, as well as a wider range of accommodations. We'd also say Phuket is probably the better destination for families with small children.
On the negative side, we have to point out Samui's chronic water shortages. While the average visitor might not even be aware of it, water shortages are a constant problem for the island. Yet it seems that development is continuing unabated. Both islands can seem over-developed, but you can also still find quiet places away from the crowds in either place.
There are lots of other places where you can find plenty of surf and sand. North of Phuket, the latest 'up and coming' destination is the beaches near the Khao Lak-Lamru National Park in Phang Nga province. Aside from escaping the crowds that flock to Phuket and elsewhere, most people who seek out Khao Lak are divers wanting to make trips to the Similan and Surin Islands.
On the mainland east of Phuket is the province of Krabi, also rapidly developing as a popular tourist destination. There are several beaches and islands in the province, including the Phi-Phi and Lanta island groups.
South of Krabi province is Trang, another up-and-coming destination with spectacular scenery and white sand beaches. Almost ignored are the old trading cities of the east coast, such as Nakorn Si Thammarat and Songkhla. While the beaches here may not be as spectacular as others, these old cities have a lot of charm and are great places to chill out.