Well, I have never tried this plus business and I don't have the 505 anymore
I did love the 505 but I needed the dictionary (for French) and the text to speech features of the Kindle; this was the sole reason I upgraded. So, given that I currently have an iPad for reading magazines, and a Kindle too, I would go for the Libre (and did, in fact) precisely FOR the smaller screen since it would offer me a choice (e.g. for lightness when traveling) that I don't currently have with my other stuff. That said, it is definitely the more disposable of my readers and if one of the parents who hasn't got a reader yet wanted it, I would probably give it up to them without a fight.
Some questions to guide you:
- How important is it that it look pretty? I don't think the 505 is a great prize, looks-wise, but there are a ton of after-market case/sleeve/skin options available for beautifying. The Libre I have heard can use SOME Sony Pocket cases, but that is pretty much it for accessories. My stepmom ignored my Libre recommendation for Dad because she looked it up on the internet and thought it was ugly as sin, and the Kobo was all sleek and white and pretty
- Have you tested out a device in person and seen how the e-ink page-turning flash looks? Some people are put off by it, though I find you do get used to it. The Libre is a bit slicker when in use; less pauses and waiting. I also find the Sony has a very fussy menu system. Deleting a book took you through like seven screens. Again, not a deal-breaker (for me) but just one more thing to think about.
- How will you carry it? The Kobo really is the lightest of the bunch, and the only one of the three that I would toss into a bag unprotected and not worry about. The Libre is light to carry and takes up very little bag real estate compared to the 505. If you buy a 505, you will need a case.
- Where will you get content? If you plan to use books from Kobo, Sony or a public library, the Sony software is more tightly integrated with the Sony. The Libre might take some fiddling to get set up. If you plan to 'liberate' all your books before loading them, or only plan to read freebies from here or elsewhere, it is MUCH faster to get them on the Libre because you can just dump to memory card.
Honestly though, I really think they are both fine devices. I don't think they are bad choices. There is no reason I would tell you NOT to buy either of them. It's just which one calls to you more