I think that the basic difference lies not in ontology, but in elective affinities, to use the Weberian term. Both race and class (=producers of commodities) have the same ontological properties of being universals in the philosophical sense (=concepts expressing essential properties of a class of objects). However, their elective affinities (=propensities to dove-tail with particular interests and claims) are much different.
Race implies victim identity that legitimizes political claims by virtue of pity e.g. "we are victims of oppression, therefore our claims to a bigger share are justified."
Class implies contributor identity that legitimizes political claims by virtue of creating value e.g. "we create things of value, therefore our claims to a bigger share are justified."
Where I stand, the appeal to creating value is indoubtedly superior to the appeal to pity. However, the American 'middle class' seems to be thoroughly overtaken by the victim mentality and the psychotherapy approach to problems, hence the considerable appeal of victim identities.
Anyway, these are my 2 kopecks.
PS. I saw your name in the Nation's letters section in reference to the critique of the Alertman's column. I thought you would support his position, no?