have you decided how much to spend?
you'll be installing yourself, I gather?
have you allowed for a system that will phone you and other local keyholders in the event of an alarm?
If the siren goes off, your neighbours will probably ignore it, except they will complain about the noise nuisance.
Some people say it is an advantage to have a extra siren inside the house (but out of reach) as it will upset intruders. It will also wake you up if you are asleep. A sound bomb may not be loud enough.
You can look up the EN50131 series grading system if you like. Your insurers may specify what sort of alarm system you have, who it it installed by and how it is maintained, or they might not care. They might give a small discount off the premium, but this may be on the condition that they will not pay out on a claim if the alarm had not been set, which might be a risk you do not wish to take. The grading system itself is no guarantee that the alarm is any good or is suitable for you, or that the installation has been well-designed.
If you are a person who might be thought to have large amounts of gold jewelry at home on the eve of a family wedding, for example, then you will be at much greater risk of professional and determined attack.
Some of the old hands that Europlex mentions are not mentally attuned to the concept of the low-cost, simple, DIY installation.