I normally go via SIN, but for my next trip in a few weeks I'm trying BKK. The main reason is that the flight LHR-BKK is shorter than to SIN. This also makes the second leg to SYD longer which I'm hoping will let me sleep more and hopefully avoid jet lag.
Coming back, via SIN works well. You stay up until about 1 or 2 am "body clock" time in SIN and board your flight, eat and then I've managed to sleep for 8 hours out of the the 13. No jet lag and last until 9.30 - 10 most trips back since they introduced the beds. Prior to that I'd need a few hours sleep as soon as I landed!!