There is but one reason to transit BKK, and that is when flying TG F. In which case, book a seven-hour layover. Suvarnabhumi can have a brutalist undertone -- literally, with all that concrete and figuratively, by googling "King Power Duty Free scandal".

SIN T1, while showing its age somewhat, is preferable for me, although I usually connect on to PER instead of continuing on to SYD. The QF/BA Jlounge is very nice indeed with funky design and nice showers. Flounge has its charms, but it gets a mixed review here. Duty free at SIN is a bit of a faff with the security check at the gate (often it will not be possible due to time constraints), but then again one can get it on arrival in Australia.