All very good points nemofly, but you're missing one big point:

9) If you're returning back, TG arrives at the same time as a bunch of other flights in the evening bank, so the lines will be long in immigration. SQ arrives at a quieter time, and you'll be out the door sooner.

Do they still serve champagne in the back?
But if you're flying to BKK, why not just go TG and save yourself a stopover?