I can't believe some of what is said by people on both sides of the NoSQL arguments. Discounting use of RDF data stores, almost all of my recent work involves PostgreSQL and MongoDB. I think that it is blatantly obvious which to use in specific circumstances. I have not had to do this yet, but using Datamapper.setup, you can integrate the use of both in the same application by storing some model data in a relational database and some in MongoDB, as it makes sense to do so.