I use the cheap (3 for $5) i-Blason protectors for my Note 3. I don't see any difference between then and the expensive Zagg on my wife's Note 3. And after over a year, my cheap protector looks the same as the day I put it on - and I use my S-Pen all the time. Looking almost flat against the surface I can't find a single scratch from any angle.
If you wipe the screen when there are corundum particles in the air, that would be different, because corundum particles are very small and just slightly less hard than diamond, so they'd cut through plastic easily.
About the only difference would be using a tempered glass protector (although corundum would scratch it too).