Fast mode vs Quick Refresh?

On the Reader, I can choose Best Display if I want a crisp display (long reading session or graphics etc). But much of the time, concerned that might shorten battery charge, I switch to one of the others. I'm at a loss to understand the difference between Fast Mode and Quick Refresh. I haven't found ghosting to be much of a problem in either. Are there pros and cons? Are there really any reasons why I wouldn't choose Best Display as the default?