Paper for manga and the occasional Bryan Lee O' Malley comic, I just feel more motivated to read them when I have them physically and...don't, otherwise.
For books that are required for some reason (ie classes, work, etc.), ebook, all the way. If I can save money obtaining a book I don't particularly want or probably won't use, I will. There really isn't a downside to it, and that only becomes more true as time goes on.