Zero 2. It's a faster CPU, which does make a difference for something even as simple as a thermostat.

Perhaps more importantly, the presoldered headers are only* compatible with the clicky edition of the #HestiaPi. The silent edition solders the headers on the other side.

*Technically, I could leave off the female headers and a pi with pre-soldered headers could be soldered onto the silent edition, but then it would not be easily replacable. So if it ever broke, it'd be very difficult to replace.