There is an excellent "spreadsheet" done here in the forums which will give you an idea of the capabilities of the various e-book readers.
https://wiki.mobileread.com/wiki/E-book_Reader_Matrix
Given that Waterstones now have the Sony Reader in stock and available to play with in their shops you can go and try one out. I'm not sure if W.H. Smith have the Cybook Gen3 available to be tried out in their shops, but the price on offer at the moment is excellent.
Calibre available and superbly supported by Kovid Goyal on these forums will allow modification of mobi files to the file type use on the Sony PRS 505:
http://calibre.kovidgoyal.net/user_manual/faq.html
Enjoy your search for your e-book reader