My wife and I went to Thailand for the first time last December after going to Goa 6 times in the last 8 years.

Had 3 nights in Bangkok which we loved then 10 nights in Phuket at Karon Beach.

Karon Beach is not too busy and you always have Kata Beach next door which is even quieter. Although quieter than Patong there are still many bars and restaurants to enjoy at night and if you want a more lively nightlife Patong is only a 5 minute taxi ride away.

We found the standard of the accomodation and the food to be better than Goa and are already planning to be back this December.

We stayed here in Karon http://www.pacific-club-resort.com it's around a 10 min walk to the beach but I would highly recommend it and great value to.

Koh Samui is good as you can get lively nightlife in Chaweng. If you stay in a resort close by you can get the best of both worlds.
Worth considering is Bangkok after your beach resort. Bangkok can be a bit overpowering when you are jetlagged and there for the first time. Also it does seem to extend your holiday as, not sure about you, if I am on the beach and I'm going home from there all I do is countdown. When you are in Bangkok for your last few days you don't have time to think you only have a few days left