Buying an eReader that comes with its own eco system is a really bad idea. I'm looking at you, Kindle. I'm also looking a little bit a you, Kobo.
Having their own book store, manufacturers are not interested in you buying your ebooks anywhere else, and they are going out of their way to make handling third-party files as awkward as possible.
Sure, you can jump through a couple of Calibre hoops in order to sort your side loaded books by series, but generally speaking this turns out to be a major PITA.
On the other hand, manufacturers that earn money selling hardware usually have an interest in making the use of their device as easy as possible. Especially, when they don't know the format of your books.
A big thumbs up to PocketBook who feature the best metadata support. Hands down!