I own both. (actually two tempering machines AND a firemixer)
the most important difference is that the heating element in the firemixer heats the chocolate ONLY on the bottom via an element that can get very hot, whereas the tempering kettles use heaters submerged in a water jacket to ensure the chocolate never burns.
The second you overheat the chocolate on the bottom of your firemixer (and it CAN happen), you will destroy ALL of the chocolate you have put in it - a very costly mistake.
Another significant difference is that the firemixer agitator turns at almost twice the speed of the one in the chocolate machine - which will most likely cause bubbles in your chocolate.
I've always believed in the right tool for the right job, and would never consider using the firemixer to heat and temper something as delicate as chocolate.
Brad