I took the train to Ayyuthaya from Bangkok for a day trip. It was a long daytrip, I agree, but it was comfortable and I had no problems getting there or back. If the prices are the same, and if they say they take about the same amount of time, I always pick the train. Much more comfortable...you can get up and walk around if you want...better scenery looking out the window...and a smoother overall ride.
However, I haven't gone as far as Chang Mai on the train, so I can't say "yay" or "nay" for that distance.
Have fun. I wanted to get up there, too, but instead decided to squeeze in a couple days in Angkor Wat, Cambodia, what with the direct flights and all from Bangkok to Siem Reap...
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"Life is not measured in the number of breaths we take, but by the places that take our breath away..."