It's an interesting question. In my opinion, it's probably more serious to have the ground station compromised than the onboard software. The Predator is just a drone and the onboard software is going to be pretty dumb, so even if an attacker were able to take it over completely, it couldn't actually do much besides crash the plane or launch its missiles. (That sounds serious, but it really isn't. If you're using a drone in the first place, it's safe to assume it's somewhere where you don't have many friendly forces to begin with. That might change if the US keeps turning into a police state, but that's another discussion.)

If an attacker compromises the ground station software, on the other hand, he gets to try to snoop on whatever intelligence information comes down from the network that computer is on. Things like lists of likely targets for the operator to watch for, or anticipated positions of friendly units in the area, and so on. You get the idea.