While em is relative to the font-size of its direct or nearest parent, rem is 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.
Nobody was born understanding CSS. We all struggled our way through the confusion. Good times, good times. Let's think back on those confusing moments, that are probably confusing people as we speak. I'll list some that I remember and you list yours.