Messaging Gateway or Mail Security for Microsoft Exchange

We propose to implement antispam and antivirus protection for MS Exchange. We find both "Messaging Gateway for Small Business Edition" and "Mail Security for Microsoft Exchnage" providing the features to implement the protection. Both the products claim to stop 99 percent of spam with less than one in a million false positives. 

Could you please guide which of the two products is suitable to provide protection for our MS Exchnage with around 150 users ? 

Thanks in advance.