The larger 9x12 coil is the same as the stock coil, just bigger.
It's like a stock coil on steroids.

Good for larger areas where you want to cover ground quicker.
Also gets 2-3 inches more depth than the stock coil.
The only drawback is in busy or trashy areas, it sees more targets
at once, and can make finding the targets a bit more tedious.
Trashy or busy areas is where the sniper comes in. It's great for
busy areas as it's only 4.5 inches and sees less at a time.
It still goes pretty deep for a small coil. About 5, maybe even
close to 6 inches on say a dime. Depends on the ground.
The stock coil is a good average size coil, and does pretty well.
I have all three, but still use the stock coil at times.
Just depends where and what I'm hunting.
I'll often still use the stock coil if I'm in semi brush, and want more
depth or coverage than the sniper, but not enough room to really
swing the 9x12.
As far as buying extra coils, you have to consider where you hunt
the most. If you run a lot of busy or trashy areas, get a sniper.
If you hunt open fields, relics, consider a 9x12.
To most, I recommend the sniper first, as it's totally different from
the stock coil. But like I say, if you hunt large fields, you might want
the 9x12 first. What you do matters more than what we do...

I got both the extra coils so I can be covered for most any kind of
hunting.
The Ace 250 with all three coils is a very versatile setup for the price
involved. You are pretty much good to go for any kind of hunting.
BTW, if you get a 9x12, I recommend a coil cover too. Will keep it
like new.
They sell a cover for the sniper, and I bought one, but I found the
sniper doesn't really need one as it has one built in. It's solid sealed
plastic. I still use the cover on mine, but it's overkill..

But I do like the covers on both the stock and the 9x12 coils as it
prevents chipping of the epoxy bottoms. I do hunt around rocks a good bit
and don't want the epoxy chipping or cracking if I whack a rock by mistake.