I’m using Gmail but I have also some other accounts from internet providers and a electronic certified mail which can be used for official communication with institutions or companies in Italy. I have also a proton like service with another address… Everything controlled with FairMail which I recommend as excellent mail client. Basically there’s no need to use one or the other but you may also use both depending of the type of message, I guess
You may compare both as, is possible to share the same profile between the 2 apps. Actually I rely on Thunderbird, as I use it since the beginning, and it has the functions I need. Perhaps I will give a try on the Betterbird as well.