That said, I would suggest picking two data stores like PostgreSQL and MongoDB and write some code examples, play around with data models for your app. If you spend several hours doing this then you can make a better decision.
BTW I have tried most of the NoSQL data stores and MongoDB is probably the easiest to use from a developer's point of view. I have used bindings for Ruby and Clojure and being able to store things like Ruby or Clojure maps directly in a documents is great. That said, PostgreSQL with built in text indexing support, etc., etc. is also a great tool. I think that if you become fluent with PG and MongoDB then that is sufficient for a wide range of problems. For my development, I toss RDF data stores into the mix, but that might just complicate things for you.
Anyway, pick one of each, have fun playing with each one before making a big commitment.