If you can get out of the airport without changing money, hold on to your money. You are very likely to get a higher exchange rate outside the airport. Bangkok usually offers higher rates than the other cities, even Pattaya. So you may want to change your money in Bangkok before leaving for your next destination in Thailand.
Air Asia is a low cost carrier, which means that their fares are usually lower than the fares of full-service airlines, Thai Airways for instance, but sometimes the airfare alone should not be your only deciding factor. Check their Bangkok airports for example. Air Asia may use Don Mueang Airport for its Bangkok flights. If your connectig flight is at Suvarnabhumi Airport, then Air Asia may not be your better choice.