AFAIK, clay will remove the contaminant "above" the paint, like poopoo bird, overspray, bug, light tar, etc...
but the pre-wax will remove contaminant "under & among" the paint (i cant find the good word for replace the "among" word in my dictionary) like a stains, acid rain ecthing, veryfine swirl, oxidation, etc...
sory my english is terrible