I'll be there in Feb.

And after some searching on the 'net, a flight on Air Asia or Nok Air is comparable to the train, just as you said.

We'll be there in Feb, so we're thinking flying would be best. We're still doing a Monday to Monday plan and looking at flying up to Chiang Mai on Friday morning.

Would it be too short to only stay in Chiang Mai until Sunday morn/afternoon? Since our flight out of BKK (to MNL) is on Monday at 2pm, would it be advisable to fly back from Chiang Mai to BKK Monday morn and wait at the airport until our Manila flight? Or should we come back Sunday?

We want to see something other than Bangkok and we are interested in seeing Chiang Mai; I guess it's just a matter of how to break up the trip. A Chiang Mai visit in the middle of our stay seems odd, we both thought we should either hit it first or at the latter end of our time in Thailand, especially since we only have 8 days.