They are massively more resistant to jamming. The antennas are very good at rejecting signals that are coming from the wrong direction, that is, not from above.
This is the correct answer, phased array antennas have much more directivity so they are much harder to jam with a ground-based jammer. Although I suppose in some cases you might want to put the jammer on a drone anyway, to cover a larger area. In that scenario you would probably not see a significant difference for phased array antennas vs "normal" antennas.
Caveat btw: for both starlink and GPS, the satellite you are talking to will not always be "up". For GPS in particular, it is possible that some of the satellites are only barely above the horizon. So an antenna that only looks "up" is generally not what you want anyway, which makes ground-based jammer more effective again.
Phased arrays are good at rejecting incidental interference, which is why they were used to address radar jamming. Of course that also meant that billions were poured into developing technology to counter that resilient property. Anyone capable of blocking GPS on a non-trivial scale would be easily capable of blocking Starlink as well.
It depends strongly on how the phased array is being steered. If the components prior to phasing and summation are saturated by the interference then phased array beamforming will not reject the interference.
