There is not a great deal of difference between SIN and BKK. The airport in SIN is more logically laid out and smaller but neither one is any more of a challenge than any other major airport. Are you planning to stopover in either city? In this case it does make a difference and, while Bangkok is a spectacular city, does not lend itself to a quick 1-day jaunt. Singapore is close, clean, and straightforward. Think of it as Asia-light. If you are there just for the sake of changing planes, go with the shorter connection time (which I believe is SIN).