I suppose in any realistic scenario we should assume that the enemy may be listening to all our communication at all times. This is the assumption behind such daily things as WPA3, SSH, TLS.
Yes, so all in all for satellite vs. fiber in backbone applications, I'd say that it's a wash (or slight win for fiber) when it comes to security, and a definitive win for fiber when it comes to jamming resistance.
In the field it's a completely different story, of course – you can't always pull fiber (although it does appear in unexpected scenarios, such as fiber-operated UAVs or torpedoes).
Wire-guided drones and missiles seem to be increasingly common, probably due to the cost of instantaneous radio jamming being so low - that's a short-lived signal that is going to a vehicle near your adversary. Torpedoes make a lot of sense for other reasons because radio has a very hard time getting through water (and things like sonar have too low bandwidth as well as giving away your position).
However, it's very easy to cut a fiber in a way that is hard to repair. Fishing trawlers do this all the time. In that sense, fiber can be "jammed" (sabotaged) much more easily than radio/satellite.
FHSS[1] has made jamming difficult in US military communications for decades. It doesn’t make it impossible but jamming the entire spectrum is nearly impossible at scale for almost everyone. At best it would affect small areas until the US sent rf seeking missiles (HARM are designed for this) at the jammer source. Also note that modern satcom like Starlink uses AESA digital phased array antennas much like a F35’s radar. It’s so much more complex than legacy analog stuff.
It’s easily possible to jam a frequency spread signal from a satellite, assuming you can get the directionality right (i.e. you need to be in the beam of whoever you’re trying to jam, or you’ll need even more power to overcome their receive directionality, which is never perfect).
Signal strength (satellites are power constrained) and distances involved are tough.
GPS uses frequency spreading too, and locally jamming that (even the military version with a secret/unpredictable spreading code) is trivial, for example.
US military gps is encrypted and heavily used FHSS. It is jam resistant, and can’t be jammed without significant expertise and know how.
Russia has some of the best EW chops in the world (after the NSA perhaps), and they struggled to successfully jam Starlink after some defensive work was carried out by SpaceX. They use Starlink in the “sea baby” USVs that attacked Crimea just last night.
GPS uses DSSS, not FHSS. The military version has a higher processing gain than the civilian signal, but it can still be overpowered by sheer signal strength.
I'm not saying that that's trivial against moving targets in a large area (especially if they can use directional antennas), but it's still very possible. GNSS jamming is a big concern of the militaries of the world, and there's currently somewhat of a renaissance of high-precision inertial navigation systems as a result.
Jamming a stationary satellite terminal, if you can get reasonably close, doesn't seem harder than cutting a fiber (although as a sibling comment has mentioned, jammers have the nice property that communication is restored once they're disabled, unlike damaged cables, so maybe the two can complement each other?)
With some satellites, you just point a dish at the satellite and get the same data everyone else gets. With more advanced ones, you have to be in roughly the same place as the intended recipient because the satellite has different antennas pointed in different directions. In either case, it's presumably encrypted data so what good is intercepting it?
How so? I'd imagine the datacenter terminal side downlink to be much more easily tappable than fiberoptics.
There are advantages in latency and potentially availability, but even there I would imagine fiber to win in an adversarial active jamming scenario.