On the subject of spectra cord, I have a guide friend who swears by it. Since I only recently learned that this is an option, I have no direct experience for or against it.
On the subject of stainless steel cables, I have paddled years with kayaks equipped with these. They are most definitely vincible, but the evidence is that they will last with due care and attention. I know people with kayaks over 20 years old, who have never replaced their s/s rudder cables. Perhaps the ocean here is less corrosive than in the Antipodes?
Skeg cables seem to be more susceptible to damage. I base this on my observations of friends with various skeg boats. I might guess at why, but since I've only had a skeg equipped boat myself for 3 years now, I'm content to let that longitudinal study continue.
P.S. - @red kite
has worked on my boats (though not on my rudder cables) and I vouch for their service and professionalism.