I suggest getting your RSS feeds into ebook form before deciding on a device. If Windows MobiPocket Reader does what you want, then this argues for Cybook, although Calibre only adds one extra step for the PRS-505. If Calibre is your choice, then the PRS-505 may be best although again Calibre will soon have a MOBI export option (and, as zelda_pinwheel said, Windows MobiPocket Reader or mobigen.exe can do ePub to MOBI today). The output of MobiPocket Reader can be tested (viewed) using MobiPocket Reader. The output of Calibre can be viewed using Calibre, or
Sony's Windows ebook Reader.