What encoder resolution?

I want to add a rotary encoder to my mill spindle so that I can do rigid tapping. 
I see that there are choices for PPR from 100 to 600. I'm going to be using a Mesa 7i96 card.
Is there a reason to pick one resolution over another?

Doesnt really matter. 100 is enough an you will have less frequency and therefor less emf topics.

I assume we are speaking of an non hf spindle right? So 6000 rpm results in 100 rps and therefor to 10.000 ppr or 40.000 cpr. So totally easy to handle.

But in this case a 600 encoder also will work efortless.