These are two separate products.
Symantec Brightmail Message Filter is a gateway filtering product ideal for service providers and large complex architectures due to its flexibility.
http://www.symantec.com/business/brightmail-message-filter
Symantec Mail Security for Exchange is for customers that need to run filtering on the exchange server.
http://www.symantec.com/business/mail-security-for-microsoft-exchange
Another product worth considering is the Symantec Brightmail Gateway - available as an appliance or a virtual appliance. This is another gateway product, with an integrated MTA and many additional features to provide further mail security in addition to content filters - for example enhanced IP reputation and connection management, and features to detect directory harvest attacks and forged bounce messages to name but a few.
http://www.symantec.com/business/brightmail-gateway
You may wish to talk to your local sales representative about which product is best for your environment.