Storing all data in a single db versus multiple dbs in Mongodb We have an app that is basically an ERP. Some of the resources in the ERP are satellite tracked. So the information related to the resource itself is minimal, but the satellite tracker produces data every 3 minutes. Our instinct was to store the satellite information in it's own db or even it's own cluster and store the ERP information in it's own cluster.

What do you think?
Well, 2.2 will add database level concurrency (this is available for testing in the 2.1 development branch as I write this answer), so if you are concerned about lock contention between the two, then separate databases would be a good idea.  

You don't really mention what your criteria are here to make the decision, so the new 2.2 feature is the only relevant one I can think of - feel free to comment or expand and I'd be happy to answer further.