People will get disappointed once they find out purchase is needed for another set of 3 missions after accomplishing the first 3 missions. At this point, you will lose a lot of potential users. Its a better strategy if you earn through ADS as you continuously increase your users if its Free. If your app gets more users & becomes more popular, more ads will come in, which means more income for you. At the same time, you can still earn for purchase options if users want ads-free. Thats how your business model should be if you want to succeed in app business. Its the best way to utilize a good opportunity. If not only for the potential, I will rate "0".