I am starting a new WPMU network that will have real estate

I am starting a new WPMU network that will have real estate agents on their own separate site. Buyers/Sellers Agents and lenders can have a profile on the main site. The main site will be an place for buyers can learn to buy a house, sellers can learn the best practices to selling their house agents can take courses to further their education as agents and so on. Each site will have links that buyers could take a course if they want too. I need to make sure that the referring site gets the credit for the client. But I'm more concerned about whether I should set this network up with subdomains or subdirectories and which way will give the network as a whole the most benefits. I've read a lot from just googling the topic but I haven't found anything clear yet. Any help is appreciated.