[Question] Is it a better use of my time to review old material I don't feel I've mastered or to study ahead for new material?

So next year I'm taking a gap year but I want to be somewhat productive with physics. At this point have have calc I-IV, linear algebra and ODEs under my belt. So learning new math doesn't feel like the right thing to do for now. Instead I want to re-dive into David Morn's Classical Mechanics because I don't feel like I'm actually well prepared in that department (also it's good practice for the GRE). The alternative is to crack open Grifiths Electrodynamics text to get a start on E&M. But I don't know which one to focus on. I'd love to hear any input!