I dont have the book yet, but i have asked alot about both and gathered as much info as possible
Furnace: The fumes tend to kill a few monks each turn, but nothing serious to the unit.
The fumes also work wonders on opposing units, as does the ball that acts as a giant censer in combat.
It adds a nice punch to any plague monk unit.
Its also pretty light on points, so i would prefer this.
The bell is not too heavy on points, but by all accounts its too random to do much good.
The 13th spell is by all means near on impossible to cast, and chances are you will misscast while trying for a minimal effect spell which is neither amazing nor game changing.
My opinion would be the furnace, not only for the above, but also since you can use it in under 2,000 point games.