I’ve received requests for this matchup frequently, and I went into this thinking I would find the store-pick (at $45, and 120proof) beating either of the other options. Let’s see!

12yr) $59. I’ve always liked older Knob Creeks, and this is no exception.
On the somewhat light nose I get classic vanilla and oak, but also some nuttiness, cedar, and maple syrup.
The palate is predominately spice and oak char, but also has vanilla, molasses, and a hint of orange peel.
The finish is medium and has more of that orange citrus, along with some nuts, and spice.

15yr) $99. This is the 12yr, but with MORE all around, plus fruit!
The nose is much more noteworthy, with oak, caramel, chocolate covered peanuts, vanilla, and a beautiful dark cherry fruitiness.
The palate is also much more fruity than the 12yr. There is more oak, but not overpowering or tannic. There is an immediate fruity, honey, sweetness, lots of dark cherry and citrus, along with some herbal complexity, and a touch of leather. The oak is present throughout, but well integrated.
The finish is pretty long, with brown-sugar, oak, leather, and baking spices.

Store-Pick 14.5yr) $45. This Single-Barrel is remarkably close to the 15yr, but definitely tastes a little “rougher”. there is more alcohol burn due to the higher proof, and there is more oak char, more nuts, and less fruit on both the nose and palate.

VERDICT) I was surprised that my pick is the 15yr since I typically lean towards higher proof! 
The 15yr was noticeably more refined and complex than the store-pick. And both beat the 12yr. Is the 15yr worth double the price of the store pick? NO, but I do think we have become conditioned to find older Knob Creeks at an uncommonly low cost. I can’t think of any other 15yr options that are available for less than $100.

Of course Single-Barrel picks can vary widely, so I wouldn’t be surprised if there were better store-picks out there!