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.
UPDATE: Jeremy changed his tune at some point:
I use rem more than I used to. I still tend to set the font-size of headers in em, but I can see where rem would be beneficial for some headers and other text. I also use rem for most margins, and sometimes for padding too.
rem guy pretty much all around myself. But sometimes do what is talked about here in Font Size Idea: px at the Root, rem for Components, em for Text Elements, which sounds like where Jermey is headed.