It depends on the device and the type of content. Are we talking about in general or only on phones? On a large screen TV, I always choose black bars because I want to see everything the director intends and the screen is plenty big enough. On a tablet screen, it depends, if it’s cinematic I’ll choose black bars; if it’s not cinematic I might zoom in to see more. If we’re talking about phones, the screen is so tiny I don’t usually watch any movies/TV on it, but if I do then I’ll always zoom in just so I can see everything that’s going on.