It depends the use. I would separate them in. 2 categories:
1-Monitoring: Constantly receiving data and extra features, this requires to be connected from the internet, so the car sends with the use of data, constantly the location to a portal, and the other person is logged into a website. I think this requires annual fee but small usually. In this case i would recommend what the other said, about putting a phone in tbe car.
2-Antitheft: This type doesn’t send all the time information and its not based on data but on sms, it has its own gps antenna and if requested it sends the X and Y on the map with an sms, the other difference is conected with the car harness.
-firstly its hidden and difficult to access
-it is connected with the doors, so when its armed and someone opens the door you get sms
-it is connected with fuel pump, in case the car is stolen you can send sms to switch it off so even if they have gps antenna blockers at least you can stop the car, ps it stopps once it reach below 20kmh, it wont stop it in highway.
-it has the option to be connected with the car alarm/siren but i dont do it, its better to surprise them.
I used to install the second type when i was back home. But in dubai no need.