You seem like someone that potentially will buy lots of books,and therefor the Kindle will be better suited for you.
Partly because on Amazon books are about $4 cheaper than on Sony shop.
As long as you buy 10 ebooks, the price difference between a Kindle and Sony reader can be won back with the books.
The only downside I find, is that if you ever want to read generic documents on the kindle reader,you'll need to email them to Amazon Customer service.
After which they will scan them for you, and put those documents into your account.
Having many documents to send can be frustrating on both sides.
Sony reader offers the option to convert every document to the .lrf fileformat.
Any scans you have, of eg
ersonal documents like loveletters of a long forgotten lover of the past (for those who want to carry them on their reader) can be home converted.
The Sony reader also directly supports .txt and .pdf fileformat. And it supports it quite ok compared to the other models out there!
The kindle supports Mobi but does not support PDF.
PDF documents need to be sent to CS, for scanning and conversion.
The Astak mentor was originally planned for november, however other sources say production got delayed untill Q2 2009.
Same goes for Plastic Logics with their plastic logic reader.
If you mainly view PDF files, you might consider the Iliad or it's bigger brother.
Mainly because they have a higher resolution, and you can make annotations on it's touchscreen.
The con of this reader is it's pricetag and the lower battery life.
Sony reader does not support the Mobi format. Cybook does.
Most comparison info can be found here:
https://wiki.mobileread.com/wiki/E-book_Reader_Matrix