I agree with wallcraft that a 505 would probably suit you well. I've used both it and the Astak/BeBook 5" reader and find their screens to be comparable. The 505 is a bit slower at opening books, reformatting for different font sizes, etc., but once you're reading it works very well.
I'd stay away from the Sony 300. It has the same size screen and same price as the Astak/BeBook, but a slower processor, no SD card slot, no case, and the list goes on. You get a lot more for the same price with the Astak/BeBook.