For me it was the difference in overall speed and especially during annotating, shown in this video: http://youtu.be/jAkDyIvClLA
Then i searched the net and found this article about the different processors of the two eReaders:
And this fits nicely with the review.
The third aspect for me was the scribbling directly into PDFs.
The fourth aspect that sold me, are the confirmed software developments in the pipeline (yellow):