Just a note, both devices support RTF naively, neither of them support doc/docx naively. The RTF support on both devices is horrible, they don't support links, pictures, tables, etc.
They both convert the documents(DOC/DOCX) to a format they support. SONY uses their software to convert DOCS to RTF. The Kindle uses their email service to convert the doc/docx to a MOBI/AZW/PRC(their all the same formats).
The result is that you will be able to highlight and annotate the converted doc. Where as in the RTF you will not.
I don't know if the SONY 650 now supports annotating RTF, but their older models did not.
As for highlighting and annotation it's easy to do on both devices, but slightly quicker on the Kindle, on the SONY you have to go through a few menu clicks. The next question you should have asked is how hard is it to view the annotations
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With the Kindle it's slighly easier with wysper sync stuff syncs up fine and you don't have to connect your kindle. With personal files there is no wysper sync so you have to manually retreive your comments if you want to view them on the PC. There are some scripts on the net that extract the notes to a document file.
The SONY automatically syncs the notes to the PC when it is connected. But it is very difficult to extract the notes. I suppose somebody could write a script to extract the XML file, but nobody has done that yet.
Also the SONY notation system is very flaky. When I had their product I lost several of my notes and bookmarks because the XML file that stores all the book data got corrupted. With the kindle each book has it's own database with that tracks all annotations.
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