If you don't want to do the secondary MEP, and since you already have AOPS prealgebra, I'd try it out & see what you both think of it in use. You can probably go to AOPS preA after MEP year 5, or as a gentle introduction to AOPS after year 6. My youngest DD used a combination of MEP y5, Singapore 5a & b, Manga High, Zaccaro's Primary Grade Challenge Math & a variety of other materials in the year before we started AOPS preA. I'm still intending to use some MEP material alongside AOPS. One of the nice things about the secondary material is that its much easier to find a specific topic if you just want to supplement with it.