It really depends on the person. I don't notice that much difference, but for some it's a big difference.
For 6" there's the Kobo, possibly the Astak EZ Reader (BeBook, Hanlin v3). Everything else starts getting more expensive and/or adding more features. If you live somewhere that the Sony's are sold in stores you might be able to compare a PRS 600 to the PRS 300 to get an idea of the difference for yourself.
This might be of help...
https://wiki.mobileread.com/wiki/E-book_Reader_Matrix
Basically it sounds like what you want is something that supports Adobe DRM'd ePub files, the matrix above includes what DRM each supports.