I prefer k9, but that's a matter of taste. Out of the gmail affair, well, I really never saw much difference (agreed fairmail is more "standard" in the way it treats directories, but once you get used to k9, you see the benefits on its own ways).

On the gmail affair, well, the route fairmail chose to do the oauth2 authentication for gmail (k9 doesn't) is through having a google account on your phone, so even if there's benefit over, say the gmail app, it's terrible, even if you use LOS4microG or similar. I no longer have a google account, since like 3 years ago, and I recommend de-googling, but I understand it's hard for many, particularly if using work google accounts, :(