What does "they tend within and between classes" mean in 'The Irreducible Mind'?
This is kinda unclear so I'm not sure my reading is the correct one. But for me the two classes are the electric/magnetic class of imaging techniques and the hemodynamic/metabolic class of imaging techniques, and each class has multiple methods within it. It's a world that looks like this: {[EEG,MEG],[PET,fMRI]}. Within a single class, the multiple methods have complementary strengths and weaknesses, at the same time, the two classes have complementary strengths and weaknesses relative to each other. Within a class, you can be looking at the same type of information but have different trade-offs in terms of temporal and spatial resolution, or image quality and expense, depending on the method. Between classes, you're looking at fundamentally different types of information, so you get a different (complementary) picture of the brain's activity.
Squashing both of those contrasts down into one sentence, you have "they tend within and between classes to have complementary strengths and weaknesses"

Probably.