I have implemented in all my apps iAd. It works, it brings me some money, but I saw it is possible to implement Google Ad view element too. It takes somewhat more time to understand how to implement. That is a down side. But I can see several app uses it. What can explain its popularity? Does it pay more than iAd?
As with everything ad-related, it depends on your stats. Different types of ads are available in different countries, and there are many ad platforms that try to maximize revenue (for both you and the companies serving them).
Since you've implemented iAd, I'm assuming you're familiar with metrics like tap-through rate (TTR), fill rate, effective cost per mile (eCPM), and impressions. These are important and will determine your revenue on any platform.
That said, Google ads are very popular not just in iOS apps, but across the Internet, which is important. Since Google tracks users' search histories, it has a wealth of information about that user, which you'd assume would bring more relevant ads, bringing you more revenue. However, Apple has data about what apps users have downloaded, which can often be quite relevant.
It's your decision to make, but there are trade-offs for each; while Google may serve more relevant ads, for example, Apple holds advertisers to a higher design standard, resulting in higher-quality ads.
A typical set up is to have the app serve ads from one platform and to use the other in case of failure (i.e. if Apple can't serve an ad when you want one, try Google).