Google Site Search vs Google Custom Search

A Custom Search Engine allows you to create a search engine and host it on your site using the Custom Search element. It’s entirely free.

Google Site Search is designed primarily for businesses looking to create a highly customized and Google hosted site search solution. For a fee, Google Site Search enables customers to enjoy technical support, customization of search results using our XML API, a tailored look and feel, optional Google branding, and no ads next to the search results.

If you have a Google Site Search engine, you can create a business group and add other search engines (including free search engines) to it. All search engines in the business group can share your Google Site Search benefits, including tech support and expanded query quotas. Create a business group.
Feature Custom Search Engine Google Site Search
Look and feel
Customize results look and feel using XML No Yes
Ads on results pages Yes Optional
Enable AdSense Yes Yes
Google branding on results page Yes Optional
Technical support
Access to support forum Yes Yes
Email support No Yes
Search scope
Search the entire web Yes No
Image search only No Yes
On-demand indexing quota 100 URLs Flexible, depending on account type