I am a Thunderbird user on desktop but I had been using Blue Mail on Android for several years and had been pretty happy with it. I may switch to Thunderbird when they have the sync feature implemented.
I used blue mail for years and recommended it to my wife and some of her friends. Everyone likes it. I, however, moved to fairmail and been very happy with it. Reason: its foss and much better than k9. Have a look at its feature set and UI.
K9 already exists today if you want to switch which is "thunderbird for android" with a different name, but what do you mean with sync feature? That it shows notifications, or fetches all your email for offline reading or so?
Basically, Firefox Sync allows you to sync certain things across your devices (bookmarks, tabs, etc) and they are going to implement similar functionality in Thunderbird.
