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.
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.