I live near Washington DC and am building a 20"-high sitting wall, faced with stone. It is ornamental for sitting only, not a retaining wall of any sort.

Is 24" footer necessarily better than 18"? How can I determine if concrete w/ cinder block core VS. rubble footer is best for me?

Thanks very much for any help.

If this is ornamental I would skip the footer entirely. I can only picture you are trying to recreate something you have seen in rural areas near DC. The folks back them did not build footings for their stone fences, they just dry stacked them on firm soil. You will have an authentic rustic fence. If this wall can not be dry stacked and needs mortar then my suggestion is not for you.