A wired system is more expensive depending upon the installation. Lengthy runs of wire across a complex building add substantially to the labor costs. Once a wired system is in place, it should be functional for a very long time. Because the sensors are just contact points instead of mini-transmitters they have no electronics to fail or wear out.
Wireless systems allow sensors to be placed without regard for how the information will be transmitted. This gives much more flexibility in locating a sensor where it is needed, as opposed to where it is easiest to place. A wireless system can cover much more area as a consequence.
The best systems combine the strengths of a wired and wireless system. Major points are wired, while more remote locations use wireless transmitters. If someone wishes to break in they need to overcome two separate types of sensors instead of just one.