Well it is pretty much an unaswerable question as you have put forward two totally different options.

I like both places so do not think you can go wrong with either. I think there is something to be said for going to Myanmar quite soon before it becomes too westernised.

I think it will also depend on what time of year you are planning to go. Both in terms of cost and also what is suitable do you want to lie on the beach during the rainy season.