Theoretically, solid state amplifiers would have a longer service life. Vacuum tubes will always have cathode depletion causing them to degrade eventually (although this is pretty well controlled in most cases for long life). In space though the answer might become more complicated. Because the vacuum tubes are more tolerant of high voltages, they are probably also more tolerant of charge build-up and discharge from free electrons in space. Significant amounts of free electrons trapped in the second Van Allen belt that affects satellites in geosynchronous orbit (like a lot of comm satellites with big TWTAs). Less of an effect in deep space though.
That makes sense, also it's likely solid state devices would be more susceptible to cosmic rays, high velocity protons, which could cause catastrophic (instant) failure whereas TWTs would be unaffected.
I wonder if you may know any answers to questions I put to AllanYx ?