The Astak and BeBook 6" devices are both Hanlin V3s, and both can use exactly the same firmware (i.e. they are identical, except for the name on the front). A big advantage of the Hanlin V3 is that it can run OpenInkPot (has its own forum here), which is perfect for your reading preferences, and there is also enhanced versions of the standard firmware available. I own both a 5" and a 6" EZ Reader, and I use the 6" with OpenInkPot every day. It is a great reader, if you can get one for a good price. The Hanlins come with several fonts, but using the standard firmware changing the fonts for ePub is a bit harder than on the Sony (see below). It can be done though, by adding a couple of lines of CSS to each ebook. OpenInkPot uses FBReader, which allows user provided fonts.
On the other hand, I am surprised that you think the PRS-505 screen is substandard. The PRS-500 had a relatively poor pre-Vizplex screen, but the PRS-505 was the first Vizplex device. I have not seen a large difference between Vizplex screens (which all come from the same production line), but the PRS-505 has the reputation of being the best EInk screen around. Perhaps it is the fonts. These can be changed, see two of the sticky threads in Reader PRS-505 Dev Corner
, one for ePubs and one for LRF/RTF fonts. Overall, if you can get a better deal on a used PRS-505 it will likely meet your needs.