Assuming your reading DRM-free PDF?
Then I would recommend the Iliad over the SONY PRS-505.
I have a PRS-505 it read PDF great "IF" and that's a big if the PDF is formatted for the 505. Once you get to larger formats Letter/A4 then the reader must re-flow. Whith single column text this is not an issue. However with pictures, graphs, and equations. The reflow does a bad job.
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