Public Relations is known as “earned media”. Advertisement is known as “paid media”. PR means persuading journalists at publications (ie. media websites, blogs, and digital magazines) to write articles about your product. Many publications are very niche, so any product has a chance of getting coverage. Advertisement is known as “paid media” because you are purchasing an Ad which a certain amount of people will see, guaranteed. The guarantee of Advertisement comes at a high price: low conversion rates.
You cannot pay journalists to write about you (unless you are bribing them which is a rare possibility). Thus, PR has an inherent risk. If you have no tolerance for risk, then stick with advertisement. But beware – you pay a high price for refusing to take risks because advertisement has a lower conversion rate than PR does.

Fast Company’s notes on PR vs. AD conversion rates: