The Google Mini Search Appliance is designed for the needs of small and medium-sized businesses and thus provides a cost-effective search solution for up to 300,000 documents. It also provides support for multiple collections, access to information in business applications through the Google OneBox for Enterprise feature, and support for user-level document security using NTLM and HTTP Basic Authentication and Authorization.
The Google Search Appliance is designed for larger businesses and can support up to 30 million documents. The Search Appliance includes a number of unique features, including direct database indexing, a feeds API allowing you to push new content into the index, query stemming, RAID (to provide continued service in the event of hard disk failure) and deeper support for user-level document security, including a security API. These features are unique to the Google Search Appliance.
For more information, see the comparison chart at: http://www.google.com/enterprise/search/compare_intranet.html