The older Middle School book is more competition-oriented. It is similar to AOPS books. Apart from the organization/prose that's optimized for MA math teams, I think of the biggest difference is that my book is more designed to be read and you could skip doing the problems whereas for AOPS books you really have to do the problems as you go along.
The Elementary School book is similar in philosophy -- you read it and it explains things -- but the prose is all about the joys of doing hard math. Unless kids are also extremely advanced verbally it's probably more a read-with-your-parents book. For kids who don't love reading and/or are impatient it can also be a text for the parent -- the kid just works on the workbook worksheets and the parent who has read the text can explain things when they're stuck.
Mostly I think kids just need to learn math up through Calculus BC in high school because they will take the other things they need in college. I'm not a fan of the AP Stats curriculum and also don't see a whole lot of value to learning multivariable calculus while in HS. Many students who go on to graduate school in economics major (or double major) in math as an undergraduate.
If kids are able to do more in middle school/high school I do think it's valuable for kids to get the deeper knowledge of high school math that you get from doing contests like the AMCs. And while this is appropriate for fewer kids, I also think it is also great if kids can get some exposure to abstract math and proofs while it high school.
I am a big fan of learning to program in HS. Kids won't learn the particular languages that economists use, but the skills they learn will translate. When I'm hiring research assistants I care more about programming skills than anything else. You can obviously put this off and learn programming in college, but it can be intimidating to go into a college programming class if you don't have prior experience and a lot of kids end up never learning.
