Check out FileMaker's own System Requirements page ( http://help.filemaker.com/app/answers/detail/a_id/13180/~/system-requirements-for-filemaker-server-13 ) for the details. You will note a table for "Database Server (Minimum and Recommended columns) and below that a section on WebDirect.
Basically, both need good networking, the WebDirect machine needs processor grunt plus RAM depending on the number of users. Since the database server machine (the Master) accesses the data, it would benefit most frorm having SSD storage. The disk requirements for the Worker are minimal - it just needs RAM and processor.
It depends on where you expect the highest load...