I have flown both.
- Seat: I like the TG seat on 380/747 better. More privacy, a bit more space. Avoid the 340 seat (worn out, not much privacy). The SQ seat has one disadvantage: you need to get up to fold the backrest of your seat down to turn it into a bed. No good way of lounging and then sleeping without getting up (and waking up).
- Food: It is airline food, reheated, typically dried out and hit and miss on both.
- Lounges: The F check in and lounge in BKK wins.
- Connections: TG wins with a non stop
- Airline reliability: With all the uncertanity at Thai and the constant equiment changes SQ wins.
Even at the same cost I personally would take TG F over SQ C.