I would choose SIN simply because the transit experience is so easy. The KF lounges are not brilliant though but in T1 there is a TG lounge but it's only open at certain times of the day but that's the only option as a *G to the KF lounges. FWIW the SQ business lounges aren't that brilliant either and SQ trades a lot on past reputation as something special which these days it isn't.
At BKK you obviously have the TG lounges as well as an SQ one but I think there are two better options, the Eva lounge and also the new Turkish one, both are way better than any Thai lounge and definitely better than the KF lounges in SIN. You are however transiting BKK and as ever, the wonders of Amazing Thailand can strike at any time though generally BKK is OK - it's just not Changi.
The TG 787 is 9 across in Y as is the SQ 777 so the latter has to be more spacious. TG service is fine, it's not as polished as SQ but IMO the food is often better but that's personal taste.