I like noodle's comment here, so I won't add anything in that direction.

What I will say though is that paying customers give you a better understanding of the actual value you are providing to them with your product. Shelling out for something isn't something most people take lightly.

On the other hand, advertising is much more subjective, and it will rely not only on what you're doing for your users, but what you're doing for the advertisers in terms of profits. It's a much more delicate balance to maintain.

Just food for thought.