Can someone give me the practical difference(s) between 12V and 24V fire alarm systems?

I'm not aware of any major manufacturer of fire alarm systems that use 12V.

I've never seen a 12 volt system. I have only dealt with two voltages on F/A.
Did repair work on an "old school" 120 volt system and install lots of 24 volt.

I don't think I have ever seen a true fire panel with 12vdc output, most of the time a 12vdc panel is a residential burg panel with a fire zone. It is a bit easier to get parts for a 24vdc system, there are very few companies that make a 12vdc horn/strobe, System Sensor is the only one that I can think of off the top of my head. It is also easier to get 24vdc smokes. Is this a residential or commercial installation?

24V system will allow a greater selection of devices, and also allow more devices on each zone.