Google Duo on the other hand, is available both for Android and for iOS-based smartphones. This makes it a great choice for people who want to converse but have phones with different OSs. The downside to Google Duo is that it’s only available for smartphones, whereas FaceTime can be used on tablets and Mac.
Another aspect when it comes to using these two apps is related to the fact that FaceTime requires you to have an Apple account, whereas Google Duo only needs your phone number.
As far as video quality and other features are concerned, users must know that Google offers far better video quality than FaceTime and by that we mean that the video calls don’t freeze or drop. 
At the same time, Duo adjusts the quality of the video if the network connectivity slows down so that it can maintain a stable connection.
Google Duo also offers something unique called Knock Knock which enables a user to see live video of the caller before they accept the call. Not only does it show you who is calling but it can also be a means for the caller to let you know if they want to show you something.
It may seem like Google Duo is the winner but FaceTime has a perk that Duo doesn’t which is the audio call feature. When making call, we don’t always want them to be video calls, but it’s great if we can opt for an audio call instead. Duo doesn’t offer this option at this point.

Source: Arab News