From my anecdotal experience, and having a B&N store close by, which we visit from time to time... and before becoming interested in the EBR world in November as I contemplated a purchase for a child gift... I would have told you that I've heard of Kindle and I've heard of Nook. But as to who sold what, I had no idea. As far as I knew, B&N sold the Kindle and Amazon sold the Nook. So my experience would refute that Amazon's Kindle defines the market in name recognition. Maybe I'm the exception.
As I began my research into EBRs, I held no preconceived ideas about one or the other. And when the $99-$129 Nook refurb deal came along, that piqued my interest in getting one for me too.
But I held off purchasing one until I could determine which of the two devices I actually preferred. And then all the refurbs were gone. Which made my choice easy as I much preferred the Kindle keyboard to the virtual KB of the Nook (and numerous other preferential reasons too). And while the Nook let me natively get epub library books, I learned how to put library books on the K3 before I even owned one. And then I discovered how long the waits were for library ebooks in my area.
That native library advantage disappeared (not that it mattered anyway).
So perhaps I'm an anomaly.. perhaps not. They are both fine EBRs. I hope they both succeed. But one has features and ergonomics that satisfy me more than the other... personal preference only and YMMV and all that.
My daughter and I (both with K3s) enjoy books from where ever we choose to find/buy/borrow them.