The Free vs Paid debate has been raging among developers for quite some time. Some advocate the paid model where a user will pay for your app and get all features. Others advocate the free model where you can appeal to a wider audience and get paid via advertising. There is also a hybrid model that is used by some where the user is shown ad's, but can turn them off by paying through an in-app purchase.
What is the best solution?
I have experimented with all three options and have found that deciding on a revenue model is app dependent.
Free apps is by far the most used revenue model. Free apps are typically supported by ads. You can appeal to a much larger audience by offering your app for free.
Paid apps must have something unique to offer for users to pay for it. Decide on a price, but most people will not pay more than 99 cents for an app.
The hybrid revenue model allows you to offer your app for free, but allow users to turn off the ads by paying through an in-app purchase. This model offers a happy medium between the two.