Assuming Jim hasn't changed his mind, he's told us what the big choice was. It's at the end of Grave Peril when Harry goes to save Susan. Our Harry summons a bunch of vengeful ghosts to kill the vampires when he could have absorbed their power and lashed out. The other Harry does the later.
Do you have a source or cite for that?
Because while Jim told us happens "at the end of Grave Peril," the details of
what that choice is (and how close to "the end" is close-enough to count) are oft-debated; but that one looks frankly-improbable.
Here's the only WoJ I know of:
Jim: It’s going to be a different character because it’s going to be Dresden as he would have been if he made one choice differently, and the fallout from that effect on his life...
In this case it will be the big decision at the end of Grave Peril.
Exactly
which decision is "the
big decision" has AFAIK never been disclosed. As I say, I'm happy to be shown to be mistaken, if you've got a WoJ citation!
Why do I find your "Harry absorbs the ghosts' power" proposal so improbable? Because that would be Harry performing a mini-Darkhallow; just pulling the concept and the methods out of his hat (a hat, let it be noted, that
Harry does not wear!) in the spur of the moment (and getting it right!) based on essentially zero experience with necromancy. Later, we see how a pile of very-experienced professional necromancers hadn't been able to figure it out, all vying for the lost
Word of Kemmler. Harry is already too much of a Special Snowflake character; if he was also out-necromancing the professional necromancers, I'd find that far too OTT (just doing a mass-summons that way is already kind of a Big Deal).
My own main theory is that "the choice" was when Harry decided to let Susan walk away, at the very end of the book. He made that choice out of selfless love for Susan, and as we've seen
Love is a Big Freaking Deal in the Dresdenverse. Had he instead convinced her to say -- applied emotional pressure or coercion -- out of his own selfishness and fear, the outcome could have been very different indeed.
My secondary theory is much darker... when Harry faced-off against Bianca, she offered him a choice: to let her have Susan (already half-turned) as "recompense" for how Harry "took away" her own preferred pet, and Bianca would consider the matter settled. Maybe Mirror!Harry accepted that choice. I don't like it as well, but that's my emotional reaction; IMO there's several ancillary details that line up very nicely, almost looking like Jim intentionally wrote-in foreshadowing elements.
