Well the 2nd query isn't really correct. Since you're already querying the release group resource, you don't need to specific release-group as the title of the queried resource is essentially the default term:
http://musicbrainz.org/ws/2/release-gro … ruction%22
Quotes mean that the whole string is treated as one fixed search term. Without quotes each word is queried independently and then possible matches are scored. Note that if you remove the quotes in the above, you will get many more possible matches.
WS "query=" searches are analogous to doing advanced query indexed searches via the website, e.g. http://musicbrainz.org/search?query=%22 … d=advanced
They both go via the Lucene search server, rather than custom web service or server search code. The syntax and queryable fields are documented at http://musicbrainz.org/doc/Indexed_Search_Syntax