subdomains vs sub folders for sub product minisites.

Hi may I ask a general question that might benefit others?

My client wants to add a directory to his site, and I'm planning to do it as a subdomain, but is that the best route?

Or should I make it a multisite and make one subdomain the directory?

AND he has 3 external sites for his key products that are wordpress, and I thought to create a multisite and copy them into it and then point the external urls. so instead of http://www.hismainproduct.com

it would be hismainproduct.companybrandname.com.
He's a small pharmaceutical company, so each med has a brand emphasis, like say pfizer makes prilosec. I think they might have prilosec.pfizer.com . From an SEO standpoint it connects the product to the brand.

Right now they are separate sites. Made sense to me unless anyone has better ideas.

I noticed ebay used to have motors.ebay.com and now its http://www.ebay.com/motors.

craigslist does it as subdomains.