Which is the better place to visit - Koh Samui or Phuket?

Neither, try Krabi or along the cost between Sarat Thani and Nakhon Si Thammarat, like Khamon or Sichon

Koh samui is my favourite only because I find Phuket too packed full of tourists and too expensive, doesn't mean you won't like it though, just my personal preference.

Koh samui has some beautiful beaches, but the majority of the beach around chaweng can be quite dirty and full of seaweed, we usually stay at the top end nearer to bo phut and the beach is beautiful there (so long as the tide isn't right in)...the other quieter side has some beautiful beaches too.

Koh samui is also close to ang thong marine park which is beautiful to go and see on a tour. And koh phangan is also close.

Phuket I find very crowded and overrated, but the main beach was still very nice,

You can also go on a tour to see phi phi islands from Phuket where they have beautiful beaches...I recommend you to take a boat tour to see all the best areas around phi phi including Maya bay and Loh dalum bay....lots of other beaches too....however if you go peak season, it will be so overcrowded, full of boats and lots of people that it may ruin it slightly for you, but upto you.

Hua hin is quite nice, not as many tourists as Phuket so can be much quieter. We took a wonderful boat fishing tour in hua hin and got to see many nice beaches.

There are lots of other places have wonderful beaches too...I hear That railay beach/bay in krabi is absolutely beautiful, am planning a trip there next, take a look at pictures on google...

http://www.thainordic.se/wordpress/?p=1225 hope the link works, if not just google it

Koh Samui

Why dont you start in Phuket for a day or two see what you think if you dont like it jump on a bus then a short ferry ride to Koh Samui, and its does not cost much

Then you can choose for yourself