You can get a good deal on Airasia, flying to Phuket. It won't cost you that much more than a bus or train ticket and will save you a lot of time.
I've recently travelled from Bangkok to Surat Thani via the night train. If you choose this option I'd definitely recommend the air con carriage! I made a massive error and got the second class train. It was 38 degrees outside and when the beds were lay out there was no air coming into the carriage. It was literally like a sauna! If you prefer this cheaper option (not ideal) get the lower of the two beds. This way you will get a draft coming through the window as opposed to being cooked alive in the upper.
Hope this helps. Enjoy your time!