How much overlap?

I'm currently in the process of reviewing a database and it's usage for renewal while looking at a new product from the same company, and upgrading another database from a different company. Same discipline, but different areas.

The information in the renewing database is specific to a certain group, but I think it is also available in other database's, but the user would have to do a more specific (advanced) search. So, I've looked at several of the other database's we have in this field to see where the overlap might be, while taking into consideration what is offered in other database's we are considering. The questions that are coming to mind: a) how many different ways do you offer the same information (i.e. DataMonitor reports), b) is there a way to define what type of different information or feature would justify the overlap, and c) what criteria should we use to determine what we can drop, taking into account we might lose features and or information, or d) should we not lose anything and go with the overlap?

At previous job, we used Serials Solutions, thus I had access to their wonderful Overlap Analysis tool. Alas, not so here. So, now I'm stuck with the question of the best way to compare overlap of one of the databases under consideration with two other databases, which we subscribe to, offering the same type of information. This time, the database is rather specialized, so it may not be as much of an issue, but what about the next time? After using SS, I really hate think that I might have to go in and (gasp) manually compare the full-text holdings for each database. Gads. It just seems so archaic to me. But, we do what we gotta do and I just may have to.