To put it another way, you don't have to talk to a person to 'know' them to varying degrees of certainty. You can gather bits of information and build your own image with out ever meeting.
A demo with a full game unlock is a solid move, but don't be stingy with the games features in the demo. You want to show it off after all! A complete game with optional small purchases sounds risky to me if you don't already have an audience, or a team to create the side purchases. Besides you can 'fall-back' to this method if it's not initially successful.