Ideally you want a solution that can scale-out.
To maintain a responsive solution for the growing number of end users as well as increasing content, you want to be able to add systems and distribute the load as well as content across systems without taking the systems off line.
A distributed nosql solution should work well. It can hold the meta data about the content. Anything that is not searched/compared against a query, e.g. audio, video, images, etc. should go in files.
Nosql typically has relatively better lookup performance. I.e. given a key, lookup the value. If your app has queries which are not simple lookup, then SQL may be better.