Or choose technology based on requirements.

If the port to SQL was so easy and used half the storage and didn't appear to impact performance - it was possibly the better solution in the first place.

Mongodb is awesome - one of best technologies I've played with in a long while - but don't be too quick to write SQL off - especially if you have a lot of relationships.

Off topic - prototyping in Rails with Mongoid and the schemaless Mongodb underneath is frictionless - it's a step change in speed from prototyping with SQL.