There are two sides to Southern Thailand; the Gulf of Thailand, and the Andaman Sea. One is not superior to the other and both have fabulous features and sights to enjoy. There is great diving on both sides, but the weather patterns are different - always check the season before you go, and it's a good idea to heed any warnings, because storms are heavy and many places shut down in the off-season.
Koh Tao
Koh Tao is a diver's paradise. It's waters are deep, blue and clear and the marine life is phenomenal. Koh Tao is a small island so it doesn't see too many visitors at one time, and those who are there are likely for the diving. You can get certified for one of the cheapest prices in the world, and the courses are professional, up to standards and generally can be trusted - but don't take my word and please use your own judgement when you get there. This island is full of beautiful boulders, which add to the beauty and intrigue of the landscapes both below and above the water. Of course there are places you can try bouldering and climb these beasts in a beautiful setting. There are all the necessities you need to enjoy the sand and sea on this island. If I have one piece of advice for visiting Koh Tao, it's please, respect the locals and don't cause any trouble. :)
Koh Phangan
Koh Phangan is where my heart is. If I could live on any island in the world, this would be the one. But this isn't about me! Ok. Phangan is a large, fairly mountainous island with some towns and villages spread out around it, each offering a different vibe and style. It feels more like a local island. Many people associate this island with being known as the "party island", and I am here to break that image, because it's absolutely just a small part of what this island is all about. What are the parties that happen here? The full moon party! Every full moon, of course. It was started in the 80's by some travelers, and the tradition has grown so strong that it's turned into one of the biggest regular parties on the world. It's absolute mayhem, and it's a lot of fun if you're prepared for it. Locals are excited to sell drinks and food and make some money from it, and they well enjoy it too! In fact, when I attended I was amazed at the number of Thai's I saw attending. Young people from Bangkok and other cities come to experience this world-renowned event that so many people come to Thailand for, and they really know how to let loose and have a good time. There's fire shows, black light paint, multiple DJ sets on the beach, and all kinds of music you can choose to dance to. It takes up the half-moon shaped beach of Haad Rin, and there are plenty of guesthouses to crash at nearby - but definitely book ahead if you're going for the party.
Outside of the party scene, Phangan grows millions of coconuts! You will notice rai's and rai's of coconut groves because they grow a large number of coconuts for Bangkok and other areas around Thailand to meet the demand. There's a growing yoga community in the village of Sri Thanu, now popular among foreigners and seeing a number of yoga and meditation schools popping up. Vegetarian and Ayurvedic food is easy to find, as well as other conscious community events attended by international nomads. The quiet fishing village of Chaloklum is in the north of the island, and it's a laid-back treasure with a very local vibe. These places around the island are fairly quiet and not overrun with hotels and visitors. The road around the island is beautiful and not crowded, and there are lovely restaurants dotted along the main road, many international now. Phangan is a place where you can go exploring comfortably on your own and still find empty beaches. It's growing in popularity and locals are starting up more small businesses, but growth is slow and island time is respected. Phangan is a perfect place to take a rest for a longer amount of time because of the large island to explore that's not very crowded, and it's ease to get around.
Koh Samui
As Railay is to Tonsai, Koh Kamui is to Koh Phangan. Not quite, but almost! Samui is Phangan's 5-star neighbor that is far more popular with international honeymooners and resort-goers, and it's still a fantastic island with mountains, waterfalls, stunning beaches and anything you want to do. (This beautiful description goes for most of Thailand's islands, which is why I've focused on describing the type of visitor and accommodation to give you an idea of the overall tourist culture.) Samui has any kind of accommodation your heart could desire, paired with any food you could ever want. Some of the sunset bars are top-notch and the beaches are out of this world. Samui is a fabulous destination, but has also suffered some of the same effects tourism has had on Phuket, in that it is still playing catch-up to manage waste and new development as popularity increases. Visitors, please try minimize your use plastic bags and bottles!