Seems like you are drowning in choice here for social media tools within the google family. So which one and why? I have done a little research to find out this :-
Google Duo can make only video calls unlike Hangouts, which can also be used for sending text messages. Google duo can make pretty fast video calls and has a specialization in it. Google Duo is a specially designed mobile app unlike Hangouts that can be synced across different devices using a Single Googleaccount.
Also, this shows what Google are planning in the future for these apps