Although in-app purchases on average have higher conversion rates, you can also charge for your app right away. If you sell your app in the App Store, then the most important question is what price tier to pick for it. You can try to figure out what is being charged for similar apps in the App Store. Check App Annie, for example, to see how other apps are doing. Use your browser to visit https://www.appannie.com/apps/ios/top to learn more. Charging for an app upfront works well only for those apps that a lot of people are aware of, when it has a good reputation, and when it is totally clear what the app is offering. Often, there are comparable apps offered for free by competitors, so the added value that your app offers must be obvious to the user.
If your app is related to a well-known brand, or when you are operating in a niche...