I think it really comes down to personal preference but I would personally go for the aquatic pack.
I love the Australia pack's animals as well but the ones in the aquatic pack are arguably more unique, all of them except the caiman utilizing a new rig.
The building pieces are by far the most versatile, with things like recolorable mock rock and tree roots as opposed to the more highly themed, theme park-esque pieces of the others (Australia less so than the other two, but it still doesn't come close to the versatility of the aquatic pack).
That being said, again, it's really personal preference, I know of many people who love that more specialized, themed style. I hope you end up enjoying whichever pack you end up getting.