It is a easy choice for me. I spent 2 months (feb/mar 2010) in Thailand and flew coach on Thai, Air Asia and Nok Air. Paying for an upgrade is a lot of money when your hotel is only $8-35/night. I liked Thai the least but all were great compare to normal service with USA airlines.
ps. also enjoyed the overnight train between bkk and chaing mai .... did not enjoy some of the overnight bus trips.