Folder o subdomain to a parallel product

We currently have two parallel lines of business in the same company.
Line 1. a portal for creating sites under WPMU.
Line 2. a portal for publishing deals of our customers and other customers (Groupon type).

To the best of our project development we have the dilemma whether to create the structure based on folders or subdomains, type A or B.
A.
mydomain.com (portal line1)
mydomain.com/deals (portal line 2)

B.
mydomain.com (portal line1)
deals.mydomain.com (portal line 2)

Please help on the best determination to follow, regards