Personally I wouldn't go for an alarm system as such, but more of a surveillance system. Video and audio recording with motion and POE detectors. Completely passive and non-alerting to the would-be trespasser.
Alarms are fairly easy to disarm, and also VERY prone to false alarms to the point that they finally get ignored. How many car alarms actually attract police attention other than the owner getting in trouble for noise pollution? Same thing with house alarms. All they do is anger your neighbors and don't deter a thief at all.
But a video and audio surveillance system will capture thier face, voice, and actions for later prosecution. Trying to deter a criminal in mondern society just doesn't work. Prosecuting them with iron-clad evidence does.