We typically think of entities as curves, faces, bodies. So a surface body and a solid body exist at the same level of bodies. That’s why your box select is always grabbing both. I’d suggest selecting one surface body, using power select to select all surface bodies, and then placing the surfaces on a layer. Now if you want to select solid bodies, you can turn the surface layer off, or if you want to select surfaces, you can turn to solid layer off.