Which tool to choose? Try both -- you will get different "book" results from the same search in the two databases.
The preferred method for searching these databases probably is a keyword option, which allows you to combine terms and/or to search for various forms of a term. You can use an asterisk (*) in keyword searching to substitute for 1 to 5 characters, thus allowing you to expand a search. Example: environment* will find environment, environmental, environmentally, environmentalist, etc.
Here are some sample keyword searches:
Doctoral dissertations are not published books, but can be considered book-length scholarly studies.
W&L subscribes to a database which offers the complete texts of over 1 million dissertations and theses.
Described as "the world's most comprehensive collection of dissertations and theses," this database contains over 2.7 million citations to dissertations and theses from around the world, dating back to 1861. About 1.2 million items are provided in full-text (PDF) form.