I guess it depends most on where one is hunting. And what for, etc..
Usually I recommend getting the sniper first, and then a large coil if
needed. That's because the sniper is a different type of coil which
makes the 250 feel like a different machine when in trashy or busy
areas.
But if one is in large fields 95% of the time, and rarely visits trashy
sites, then it would make more sense to get the 9x12 first.
I didn't face this dilemma as I bought both extra coils at the same time.
Myself, I would not buy the large Aussie coil at this time.
I think the bang for the buck is better for the 9x12 coil. Those 3rd
party coils cost a lot. And also, so far I've seen almost zero data
on their actual performance. A couple have reported trying them, but
none have really done any real comparison tests between the large
coils, and the Garrett 9x2 or stock coil for that matter.
They say it works, but nothing much more than that..
Heck, they all "work"... :/
I wanna see some serious comparison testing before I would ever
plunk down the money they are asking for those.
At least some careful air tests if nothing else.
Air testing would give a decent indication. It's not the same as ground,
but the performance should be fairly linear between the two types of tests.
IE: if a ground causes you to lose 20% of your depth performance vs air
testing, you should see about the same reduction with any type of coil.
And visa versa if you had an enhanced target conductivity due to
leaching into the soil, etc.
So while air tests may not tell the whole story about how a coil
will do in a certain patch of dirt, they can be quite helpful when
comparing one coil against another.
But I've basically seen zero data so far on the big 3rd party
coils, so I couldn't recommend one until I do. It would have to
show noticeable gains over the 9x12 before I would even think
about buying one for the money they are asking. The 9x2 is
fairly cheap in comparison, and it can go fairly deep. It does
2-3 inches better than the stock coil on average.
I'm not saying the 3rd party coils are bad, I'm just saying that
no one has proved they are worth the extra money yet.