emis relative to the font-size of its direct or nearest parent,
remis only relative to the html (root) font-size.
Jeremy tends to favor
em, because of the ability to control an area of a design. As in, scale the type in that specific area relatively. I have tended to like
rem because of the ability to scale type across the entire page easily, but I’ve gotten into issues where that wasn’t good enough control so I could see moving back to
em for that reason.
from CSS-Tricks http://ift.tt/1huQADH