The Tip. But you should make it as vertical as possible, and free from close objects.
The Transmitted waves from the Sat come at the Antenna from all directions.
The Transmitted waves from your GPS go out as something like a water waves from a rock in the pond, in all directions.
If you have something right close to that ball that is reflecting or collecting your XMissions you could get a slow down in the Info-speed. (or worse)