1. Koreader handles pdf much better;
2. for the epub, with style regulations, it gives you granular choices on which parts of the css of the book You decide to ignore (margins, line height, fonts ecc.) And You can obtain Hyphenation even when the css explicitly avoids it;
3. if the book has images or footnotes you must use Kepub in Nickel but with Kepub You have no ligatures, Koreader handles them (Koreader can read Kepub created and invoiced by calibre, it can't read, of course, Kobo Epub bought from Kobo and directly downloaded in the reader because of DRM);
4. You have 3 ways to manage the footnotes: pop-up notes, in page foot notes, or the traditional link.
But also Nickel has its strengths:
1. excellent integration with calibre for metadata (Kobo is the best reader for that). You can manage collections, sub titles and series etc;
2. Pocket integration;
3. Only with Nickel You can read the book You borrowed from libraries (DRM);
4. this is a totally personal opinion: it is more beautiful. But really: I (personal opinion, again) don't need to know anything about the file structure of my reader, only which books it contains, so I prefer the way used by Nickel, but this is a question of preferences, many other persons prefer to have a real file manager in the reader.
I read a lot, and I alternate the various readers, with a preference for Koreader, sometimes I also use Plato, a younger but already interesting project.
But in the end: install it and try yourself if it is suited for You.
