Well, if you want to get into BKK earlier, then I guess TG635 would be the way to go. I personally though wouldn't be able to get any significant sleep on TG635 with the flight being broken up, whereas on TG657 I could get a few hours of sleep on the plane, then a few hours of sleep at a nearby transit hotel (or in the airport). All total, I'd probably get about the same amount of sleep, but that all depends on how well you can sleep on a plane. To maximize your F experience the next day, best to choose which option would allow you to sleep best so you're not too tired for the F flight.