Bala,
There are several main differences. The first is that the Symantec Brightmail Gateway is a stand along appliance which is either purchased with hardware through Symantec or an authorized reseller. The Brightmail Message filter offeres a little more leniency in where it is installed as you can position it on the mail or as a gateway. Another major difference in the content compliance on the Appliance (Brightmail Gateway). Along with the ability to import structured data and create regex based compliance polies it also allows integration of Vontu as well.
I hope this helped some. Per your other thread I am still researching to see if we have a comparison page for you.
Thanks,
John