Goncalo Alves is SO different from Koa that mistaking one for the other seems like a rookie error to me. I have built with both. I did once see an alleged Pernambuco guitar from a Canadian builder showing at Healdsburg which I instantly recognized as Goncalo Alves. It is not subtle.
I currently have on hand 5 sets of Oz B'wood from three trees, and 10 sets of Koa all possibly from different trees. Of these one of the sets of Koa is noticeably denser than the rest, but the two sets of B'wood which I believe are from the same tree (same flitch) are in another category of density altogether.
The way that the materials are hard is different as well, but hard for me to describe as it is a feeling rather that something I measure. I think the Koa has a shorter grain and a brittle sort of harness, whereas the B'wood has the feeling of a structural timber to it, long fibers and a feeling of general competence which I really like. It occurs to me that it is a bit like comparing Redwood to Spruce, structurally.
The guitars I have made from B'wood seem much more focused to me, more in the Rosewood direction. Whereas on this simplified scale of things so common to guitar talk, the Koa is almost always a lighter more felicitous sound, more in the mahogany direction.
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