*Andriod apps with ads-:Adsense, Admob, startup etc provides an opportunity to the andriod programmers to earn some cash. Programmers can provide their apps for free download while they earn from advertisements. Ads are usually in CPM or CPC rate so getting more app users means more money to the apps developer. Usually internet apps are free and use ads for making money. Best part is even if the app is not much good but has many users will get more cash. Usually beginning app developers go with this way because they can make some good money even if the app doesn't have much features.
*Paid andriod apps-:Andriod programmer ask for a little charge for using their apps. Usually this kind apps are advanced and has many valuable features. And also security researchers suggest andriod users to buy apps than looking for free download as considering about the security. Some free apps are malicious also. So paid apps has its own demand, but it is not easy to get much customers. For that programmer need to explain the unique features and many useful things with the apps. If the paid apps are very useful then it will get more customers. Usually paid apps are created by advanced programmers. For beginners it is not possible to get more users with low features. The apps rate are decided by the apps developers. Provide low price and good features will be the key point to get more users.
Both the methods are good for making money for andriod programmers. In google play you can add your apps and generate income either by ads or paid. Earning comparison of both depends upon the numbers of users it get. All the best.