I am failing to understand the difference in Impulse Sparkover voltage vs DC Sparkover voltage of GDT
Since this is indirect lightning and has a fast waveform, should the
  tvs diode be slightly above the Impulse Sparkover voltage?


If the TVS breakdown voltage was above the GDTs impulse sparkover voltage then the TVS will never operate and would be redundant in your circuit.


  Or can I just have tvs diode above the DC sparkover voltage?


If the TVS breakdown voltage was below the GDTs DC sparkover voltage then the GDT would never operate. So yes, the TVS voltage must be greater than the GDTs DC sparkover voltage and less than the GDTs impulse sparkover voltage.


  Also which is more dangerous or destructive - equipment destroyed from
  high voltage or equipment destroyed from high current?


This last part is unanswerable; equipment destroyed is still equipment destroyed hence the "destruction" factor is the same. You need to consider a specific scenario.