Normally I'd suggest the Gulf side at that time of year too...better chance of good weather. Somewhere like this on Ko Tao for example.
Well I'm trying upload a pic of the Jansom resort directly from my computer onto this thread but it's not happening as usual Admin!
Back to your question before the time limit expires...
Unfortunately you're pretty limited timewise so maybe krabi would be best. I'd stay in Ao Nang rather than Railey itself, even if ao Nang does remind me of an English seaside town a bit...promenade and c.r.a.p beach. It only takes 10mins to get to Railey by longtail and it's a pretty small place, takes less than 15 mins to stroll round everywhere. Ao Nang has better value accommodation and more choice when it comes to restaurants etc. Railey has one nice beach and a lot of longtails but nice karsts.