Probably neither. I doubt the presence of a driver today is much of a deterrent to thieves. Most theft from trucks is by people working in the warehouse.
There's not much opportunity to steal stuff from a truck except when it's being loaded/unloaded. That doesn't change whether or not it's driverless. If you're planning on hijacking it enroute, then whether or not there's a driver makes little difference. cf, carjackers.
You base this on what? Someone wanting to go to the effort of stealing the contents of a very large vehicle that is already being tracked by GPS and is for the most part in public view is not going to be deterred much by a human driver.
Scenario today: truck stops at rest stop, driver gets out, truck is stolen.
Alternate: truck stops at rest stop or for traffic light, driver does not get out. Robber gets in passenger side, points gun at driver's head and tells him to GTFO. Drives off about as fast as a loaded semi can, which is not very.
Scenario with automated truck: truck never needs to stop at rest stop except for fuel and will need a human attendant (unless we also automated those away). Odds are, there are no windows or steering wheel/human controls anyway since it's automated. Now the robber needs to unload a trailer onto another, waiting truck which is a time consuming operation. And take off with that one. All the while the automated truck is reporting its location, and that it's being tampered with.
So yeah, I don't think crime will change much. It will still be far, far easier to steal stuff from the truck when it's being unloaded. And that has nothing to do with a driver being present or not.
Not everyone has what it takes to point a gun at a driver's head (actually, not everyone has a gun in the first place). If the truck is driverless, then it removes the courage factor, the inherent risk of having to deal with a human, and now it only takes cunning and intelligence. Since it is driverless, you could even use a RPG rocket against it, without risking a human life. Maybe, I played too much GTA. That being said, I find it hard to predict whether crime or security will prevail.
I'm fairly sure if you just park your car in front of the truck on a remote highway the drive safety systems will kick in and stop the truck. Then you jump out and crowbar/bolt-cutter the container.
Of-course it will probably detect you doing that and call for help complete with its GPS position.
I also expect some enterprising think-outside-the-box robbers will temporarily come up with some creative way round this. (Will phone jammers stop the truck calling for help?)
There's not much opportunity to steal stuff from a truck except when it's being loaded/unloaded. That doesn't change whether or not it's driverless. If you're planning on hijacking it enroute, then whether or not there's a driver makes little difference. cf, carjackers.