Some are able to eliminate duplicate records, or pages on similar topics at the same address. Some will sort your pages into topics, or will eliminate duplicate pages. Some don't sort the records very well, or will only show some of the search results to save space. When you search in a gateway you are usually only searching in the pages organised by people, although some will switch to a search engine if they can't find anything in their own database. Less likely to get sites that are no longer available.
Organisation makes finding material on a topic easier.
Information is likely to be more substantial and relevant. Smaller coverage of the web. Less likely to have very new sites. Subject organisation and categories not always clear. Maintenance is very dependent on the amount of human input available.