Go for F if you can -you won't regret it. Whether it's from BKK-SYD or SYD-BKK, it's worth the miles just for the treat! I would however, strongly suggest using the miles SYD-BKK and not the other way around, although if it's a special start to your trip you're after, then obviously, BKK-SYD makes sense. Either way, just book it, I think whichever way round you do it, you will have a ball!