Sailfish OS would do so great on a tablet!

As an OS I mean, BUT with the low amount (in comparison) of native apps right now, and the poor support in the Alien Dalvik, I would not think they are ready for tablets nor phablets. A tablet requires way higher amount of native applications, at least it needs paid apps support and a fair amount of support with native games and apps.
The included document readers are a joke right now.

Why would you otherwise buy a tablet? To surf big-screen with the buggy and shaky native browser? Or use android firefox (well, then I'd rather buy an android based one with firefox running in real android).

It's the J2, without hesitation.