Mozilla's Thunderbird subsidiary even took over the project recently (maybe a year ago) and has improved the feature parity and is planning to rename it to Thunderbird Android soon. They meant to do it in December but it was postponed.
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.