Many thanks for your answers and advice.
Unfortunately, the DX is not distributed in the UK, and is too pricey for me.
Regarding the choice between a K3 and the Sony, it is a dilemma. I will not read only PDFs, so it is not crucial. Furthermore the network capability of the Kindle makes it very interesting, although I will not buy DRM books by principle (but the wireles has other useful usage than buying through Amazon).
How the reflow makes PDF reading so much better? I looked at the pictures side by side of the two devices, and it was not clear to me the PDF on the Kindle was not readable. Are there other reasons, such as the zooming and page navigation handling? I mean is it easier to change columns in the Sony that is not practical with the K3?