I personally prefer serif fonts in my books and ebooks, but sans-serif pretty much everywhere else (including manga). I think the serifs kind of make it feel more book-like to me. It's like I am reading with my pinky extended.
Meta aside:
This actually reminds me of a disagreement I have with my wife that could be a future discussion topic. When reading a book digitally, I actually prefer just vertically scrolling through the book rather than flipping "pages".