If the viewers and/or pageviews or what not are going to be aggregated for the purpose of ad sales, or traffic projections for selling the sites to an acquirer,

If the viewers and/or pageviews or what not are going to be aggregated for the purpose of ad sales, or traffic projections for selling the sites to an acquirer, I would go with a single domain with subdomains. It is a much simpler, easier story for everyone to understand and builds the main brand so much better. Been there done that on separate domains. Won't do it again.