Thank you for your answers!
Some questions:
1. With motion detectors how to you ensure that you will not accidentally activate them yourself? Will I need to disarm the security system every time I want to walk in a room with a motion detector? Similar question about window contacts, especially if we include the upstairs windows: If I want to open a window at night to get some fresh air will I need to disarm the security first?
Most alarm panels have a "Stay" mode and when you arm the system in this mode, the perimeter devices such as window contacts or glass breaks are armed, while interior motions are not armed. This way you can move about the house and still be protected.
2. Regarding Glassbreaks, how will I know that they actually work? Some kinds of glass is very sensitive and can shutter easily, while there is also laminated and other kinds of glass which is tougher. Will Glassbreaks work equally well with all types of glass?
Each glassbreak has different characteristics. Acoustic types listen for certain frequencies that are found in breaking glass. Other types are mounted on the glass or frame and measure vibrations. Most brands have a testing device that simulates breaking glass in order to test their models.
3. What about home insurance against theft? Would spending more for such an insurance and less on home security be a better idea?
Homeowners insurance protects against theft, etc. and your agent can help you determine the level you need. Some companies will give you a discount on your insurance with a professionally monitored system. Of course, insurance doesn't protect your life and safety from intruders.
4. Could some very strong deterrent measures be enough? Obviously a thief doesn't know what sensors and alarms you have in your house, but he can see things like cameras around the house. If the house looks like it has super strong security, wouldn't the thief choose a seemingly easier target instead?
Signs in your front yard, stickers on your window, cameras and any other visible sign of security will have a deterrent affect for a lot of criminals. Most are looking for easy targets and if they see signs of security will move on, but there are no guarantees.