there was already a thread with similar topic and the broad conclusion was to go for TG F due to superior on ground service in BKK (pick up, cart and FCL), DOM, Caviar and superior beverages on board of TG F, slightly better seat, usually better service due to higher ratio of FA to passengers on TG F

in your case you have to also consider
- TG direct flight while on SQ you have to go through SIN with slightly longer long haul flight and a second leg BKK SIN
- SQ need 51500 miles more than TG

of course you can make the comparison more comprehensive by finding out the airplane types and according seats you would be flying with as this matters on both airlines