I test-drove a Sony e-reader for a while, and didn't like it at all. But I really, really, really like my Kobo. Now I know the Sony design has since improved, but I'd still go for a Kobo next time.
Of course your third option is to go with a Kindle, which also compares very favourably. But if you're going to be getting a lot of ebooks from your library, you'll want to find out which platforms they support.
Going tablet has a lot of advantages too -- you can do other things with it, and you can download the right app so that you can read any "brand" of ebook. But it has disadvantages too -- aside from weight/size/portability, the battery typically doesn't last nearly as long between charges as an ereader does.