We are developing a push notification delivery service (Something like 1Signal) in Iran. We're using Firebase as push provider as Google says so, but for client library can't decide over using FCM or GCM. We wan't to report the delivery of a notification or clicking on it and also flexibility on handling notification for developers.
I've read the official docs and SO questions like this which are not very helpful and the fact that I've not found a push delivery service using FCM in their SDKs adds more confusion.
So, an specific and technical comparison would be great or tell me why should I use FCM SDK in my SDK for a push delivery system?