TXT, RTF, HTML and PRC should all work on the new Cybook.
LIT: easy to transform those into something else
PDF: A4 PDFs ? You can use Mobipocket's software to transform them if you buy the iLiad/Cybook.
Overall it depends if you have a large number of Mobipocket files. If you do, the Cybook sounds like the best choice, otherwise, the Sony PRS-500 should be fine.