For math, you need to make sure they have math facts (depth) down thoroughly well, or it will hold them up in later math classes. They need to know quickly and solidly addition, subtraction, etc. So early depth is super important, IMHO.
Later on, you can go for problem solving skills and advancing. The common core seems to repeat itself, so if you miss some concept one year, it will surface again later.
My son is taking a one year high school geometry class in 5 weeks. I'm helping him, although I don't remember a lot of this stuff! However, looking in his textbook and watching Khan Academy videos allowed me to figure out all of the questions and go over it with him. (Problem solving skills!)
