Will the travel be before 10th August? One of the runways at BKK will be closed between now and then, and this could cause delays, and may be worth factoring into your decision.

Also: if you do have the flights on separate tickets and will have checked luggage - a question worth asking - will TG definitely interline bags to SQ? I know they are both in star alliance, but I have encountered some carriers who won't interline to other star carriers if on different tickets. I have feeling TG do, I am sure others can advise with more certainty if they do or don't. (If they don't interline and you will have checked luggage - 90 mins is definitely not long enough at BKK).