I think if OP is looking to just stay in the lounge to work + shower during the transit, then given that OP will only have a *G card, BKK is the better choice. Even before the renovation started for the KrisGold lounge at SIN, the BR and SQ lounges at BKK were both superior it. Haven't been to BKK since the TK lounge opened, but have heard good things about it too.

Would agree overall though that SIN is by far the better airport, so much so that I don't really mind if I have lounge access or not there. BKK really does feel like a shopping mall with some air bridges attached. That said, went through in February this year, stuck to the lounges and it wasn't too painful. Main obvious difference is that when transiting (at least INT-INT) you go through security at a central point before re entering airside as opposed to the security at the gate system at SIN. Other issue is to make sure you know where your gate is as the walk can be a long one if it's at the other end of the airport.

As for SQ vs TG on board in Y, I'd say that SQ is a bit better, but I wouldn't hesitate to fly TG either. Main difference to me was the food is slightly better on SQ and the service overall is more polished on SQ too. If SQ is 5 stars, then TG is 4-4.5, I think.