Sounds like to me there needs to be a 3 pronged approach if you want the most security. A way to stop the car from being started, a way to stop the car from being towed and a way to track the car if it is stolen.
This'll probably stop 99% of thieves, but as mentioned, if someone wants it bad enough they'll get it.
Trying to figure out the most reasonably effective, but non intrusive, severely inconvenient way of doing so. Would stink to have one method, but it stolen from a different way.