It's a coinflip. Some users will route straight to Asia (because their ISPs are buying the right bandwidth) and be better off with Singapore. Some will route to Asia via the western US, and will be better served from California.
It's bad enough that when Blizzard sold region-locked Starcraft 2, the Australian version of the game was the only one which gave players access to two regions - Southeast Asia and North America.
Fortunately, it's 2014 and both AWS and Rackspace are in Sydney now. If you're serving Australians, I think you should just use one of them.