Concret Block vs. Monolithic Pour
A re-inforced concrete wall is always stronger than a wall built of CMUs, even with the rebar and solid fill your contractor recommends. The question is whether that extra strength is needed. Are your soils unstable, clay, or wet? Do you have heavy freezing? Is your house extra large and heavy or designed with a lot of jigs, jogs, and angles?
In many parts of the country, laying a foundation wall from individual CMUs is the standard practice because it is compatable with the local soils and climate. In other places, Re-inforced concrete is the norm.
It could be that your archy has a background in another place, or it could be that your contractor is limited in his vision and experience.
Ask around locally. Ask the contractor whether he can show you work he has done similarly ten years ago. Ask the archy why he prefers the concrete.
Finally, compare the costs for each. Then decide if the price is worth it to you. After all, it's your dinero.