Another former 911 dispatcher here. I concur with the pp, most are false alarms. However, they can be useful IF used correctly. For your purpose, to have a central monitored fire alarm might be great peace of mind.
One thing that a pp also mentioned was alarm ordinances/fines. Some cities/towns do NOT allow audible/non centrally monitored alarms of any kind. (Audible meaning heard outside of the home.) Because they can do off for no reason and be a noise nuisance to neighbors, and yet the police have no way to contact anyone to come turn it off. So be careful of this.
For alarms that ARE centrally monitored, if you choose to get motion detectors be sure to keep in mind things like balloos with helium that are loose in the house, or curtains swaying on a breezy day if a window is left open, pets that are in the home, etc. All of these can set off the alarm. And in some towns the fees are STEEP.
Also, carefully consider the central monitoring company. Where they are located, what capacity their call center can handle. Be sure they have the correct #s on file to contact for your town in an emergency. Many of us aren't aware that our 911 calls are actually answered by a regional center, or by the state police in some cases, so do your homework and be sure the alarm company has the right # to call. In a true emergency, a delay can be deadly or costly.