[bitdiffusion]

It's not necessarily all or nothing - I have worked on several projects now each using multiple database-type options: mongodb for read-intensive, loose-schema type stuff where the growth is generally predictable (e.g. products, suppliers, logs), postgres for relational-type stuff (orders) and solr for searching (I know solr isn't a database but people seem hung up on whether mongodb supports case-insensitive searching - hint: don't use any database for search).

I doubt that, unless it's extremely simple, any set of requirements are an exact match to only one of these technologies... mix and match is the future :P