Fry's currently have the 505 for $239.99
http://www.frys.com/product/5432609
But that is just thru tomorrow.
I figure you are gonna do fine either way. The 505 is a sweet reader, and the 300 looks just as sweet.
I figure based on your criteria, it just comes down to size.
Personally, I would go for the 505. Not that much bigger, but more screen real estate.