please allow me to chime in. Mind you, I do not own any of the devices mentioned, but am currently also researching getting one of these.
A quick question:
Where is he living? Europe? US? Canada? Asia?
IMO an advantage for the Kindle DX over the other two might be that it has a 3G modem built in, so you can download amazon books (and very many free books that are available via amazon) without a separate internet connection. If I am not mistaken, it even has a web browser and allows you to access the internet via its 3G card.
ePub books without DRM can be converted to PDF or MOBI (amazon Kindle format) using the free software calibre, and I guess that could be sent to the device via 3G. However, I understand there is a cost for this, e.g. 1 USD per megabyte. Anybody who owns the device can maybe comment on that.
Last but certainly not least, the Kindle DX is no longer available new, and used devices in good condition typically sell in the 250-300 EUR price range in Europe.
One more consideration: the e-ink screen of ANY e-reader is highly sensible, dropping it once can already result in the glass substrate breaking, rendering it useless. Make sure to always use a good cover and treat it with care.