Full-size PDFs are much easier to see on my Kindle DX than on my PRS-950. However, PDFs are far more functional on Sony.
More things Sony does that Kindle DX can't:- Highlight and make notes in a PDF (Kindle 3 can)
- Perform dictionary look up from a PDF (Kindle 3 can)
- Use the table of contents in a PDF
- Follow navigation hyperlinks in a PDF (e.g. footnotes, cross-references)
- Zoom to custom magnification in a PDF (Kindle DX offers only: fit-to-screen, 150%, 200%, 300% and actual size)
- Adjust PDF contrast and brightness (DX is non-adjustable, Kindle 3 offers 5 contrast levels)
- View in two-page (PRS-950 landscape only) or column split page modes
- Open a DRM-protected PDF (Kindle 3 can open DRM-free password-protected PDF, DX cannot)
- Display and sort notes from all books on the device
- Export notes to PC (Kindle has a text file chronological log of all annotations, but no per-book export)
- Create and sync collections with your computer
Some things Kindle does that Sony can't:- Run purchased games and apps
- Post links to notes and highlights on Twitter and Facebook
- Download ebooks and text files via the web browser via 3G or wifi (Sony allows Google Books downloads over wifi)
- Browse web and use Wikipedia over 3G (Sony limits use to wifi)
- Automatic document conversion and wireless delivery to device via email (not free if delivery is done over 3G)
- View popular highlights (as made by other Kindle readers) in purchased books
- Integrate audible.com library and download audiobooks directly to device (wifi models only)
- Convert text to speech in personal mobi formatted documents and many Kindle Store books (some publishers disable this feature)
- Lend purchased e-books (a limited feature)
Both Kindle and Sony offer on-device collection management. It's easier to add/subtract multiple books to/from a collection on Sony, but it's easier to add or change the collection(s) for a single book on Kindle.
Kindle is recommended for people who don't have a computer or don't want to ever connect their device to a computer.