I don't shoot much stock, but what I've done has been for a purpose. The client had a specific purpose -- we need images of X for use in large displays (high res = film), decore (med to high res = either, but film would be preferred), specific industry publications or editorial (relativily low res = digital). I've never tried the generic stock stuff you find online because it seemed to low of a return on effort. My opinion is that digital that is 12mp or higher would meet the vast majority of needs.