I had/owned all of these devices at some point. Of the three, I like the Libre best.
1) Can download right to memory card and into folders, so no need to muck around with software, tagging for collections on 505 or managing unwieldy list of books on the Kobo
2) By far the fastest. Also, small and light and pocket-sized. And it comes with a wall charger, which the others lack.
3) Cons are it's ugly, has few accessories and sometimes the battery indicator is not the most accurate. But it is a joy to read on and handles all formats.
The 505 is good too, but the Libre is a bit easier to load up and to work with since the Sony needs to be plugged into the computer and worked with via software of some kind and the Libre has the folders right there.
I don't recommend a Kobo right now. Too buggy, and some issues like how you can't group the books, which is a problem since it comes with 100 pre-loaded titles you can't get rid of.