Viewing 12 MP vs 48 MP photos on my 5K 27 inch iMac without zooming in, there is a difference, but it's pretty small, and the average person probably wouldn't even notice. However, if you crop the photos for any reason (to reframe the shot, or to make them 16x9 to use as your desktop wallpaper), the difference becomes a lot more apparent. I've found that 12 MP iPhone photos basically can't be cropped at all if you want them to still look good on a computer screen, but the 48 MP photos have plenty of room for cropping.