If you are fortunate to find good deals both ways, you may save something on Air Asia. Generally, you should book many months in advance. The problem that irks me in all the extras they add on (insurance, baggage, credit card "service fee" even on their own credit card!, etc). This makes booking a nightmare, at least until you can figure out how to get through all the tricky options. Often Thai ends up being just as cheap, when you factor all this in.
That said, the trip is not bad.