There's a $25-$30 difference between the Japanese and the Australian models. If you have the money to buy an n3DS an extra $30 to have English menus isn't much. That plus the fact that you might want to actually buy games occasionally makes the Aussie model a no-brainer IMO.
The longer you put it off the more likely it is that an update will drop before it arrives. May I ask why you're wanting a Japanese n3DS over an Australian one? Keep in mind that if a vulnerability patching update is pushed before your n3DS arrives
there is absolutely no way to update to an exploitable version. It could be many, *many* months before a game packed with the 9.0-9.2 update is released. You'll have to keep a close eye on every single Japanese release over the next few months, then have to find a reasonably priced copy to import and wait an additional couple of weeks for it to arrive.
More hassle than it's worth if you ask me.
Australian here, bought one in store on day of release. I can guarantee they'll have 9.x installed.