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.