This is my own personal opinion only. I own a Libre and I have handled a 505 at the store. The 505 does look more "solid" as the shell is more metallic, while the Libre is hard plastic. Not being used to the eink "flash", it does bother me at first. Plus it has no SD card slot as you know.
I have had no problems with my Libre so far (four months), never had to reset it, nothing. Then again, one of the big things I use it for is to download library books due to its DRM handling. Don't know if you can do this where you live, and if not, if you would not choose a larger screen instead. SD card slot allows you to load music also though the file handling is not great, just dumps it all into one directory.
Haven't seen or handled a Kobo. I really wanted to buy one when it came out, then the forums started mentioning the problem with not being able to change font sizes on certain types of epubs. Also a smattering of problems about battery life, though compared to the Libre it must get similar battery life length.
Too bad the Mentor did not work for you, I think the 6 inch screen would have been a plus if you don't plan to bring it around too much.