The CUFTS Resources Comparison Tool is a nifty little utility that allows you to compare the contents of one database to another. Can also limit to full text resources only, which is probably what you're really interested in comparing anyway. The output is a simple table that first shows duplicates held by both databases, then (scroll down) titles only held in database A, then titles only held in database B. Finally a simple tally. For instance:
ABI/INFORM Global only: 617
Business Source Premier only: 1619
This tool is a byproduct of a much larger product called reSearcher that I really should write more about one day.