For The Goddess Test, I prefer the US cover to the Australian one. It just draws my attention more. However, I still do love the Australian one. It's very pretty.As for the Graveminder, I like the Australian cover more because it's very cool. The intricate design is pretty. The American version may have more to do with the book though (but I wouldn't know since I have not read it).
For me it's the US cover in both cases :)
For The Goddess Test, neither cover particularly draws me. But for Grave Minder, I like the Aussie cover best. I think it's more unique than the american one.