For stability alone, neither head type is automatically better. A good ball head can be very stable, and a good 3-way pan head can be too; the bigger issue is overall build quality. Cheap heads of either type may slowly drift during a long exposure.

For astrophotography, though, the more important point is that a normal tripod and head are only suitable for star trails during very long exposures. If you want to keep stars or deep-sky subjects sharp over something like 20 minutes, you generally need an equatorial/tracking mount (or star tracker) that compensates for Earth’s rotation.

So the practical advice is:

• If you’re shooting star trails with a wide lens, either head type can work.
• If you want tracked astrophotography, choose the tracker first, because some trackers don’t require a separate head.
• If you do need a head, pick one with a load rating comfortably above your total setup weight (camera, lens, and tracker if applicable).