Hi Yan,
Let me clear that up for you!
compile 'com.google.android.gms:play-services-ads:9.0.2' makes use of the standalone (without Firebase) Mobile Ads SDK. That, in effect, provides the same experience as you have been getting with compile 'com.google.android.gms:play-services-ads:8.4.0'.
On the other hand, compile 'com.google.firebase:firebase-ads:9.0.2' makes use of the ads portion of Firebase. This, of course, means the Mobile Ads SDK. The benefit of using this dependency is that you can enjoy the other Firebase components, too, should you choose to compile them additionally.
Let us know if you have any questions,
Vu Chau
Mobile Ads SDK Team