Auto FP setting for Nikon D300s & D700 - high speed flash sync

The Nikon D300s and Nikon D700 have a custom setting to enable high-speed flash sync - custom fucntion e1. However, you have the option of setting it to either 1/250 Auto FP, or 1/320 Auto FP. I've often been asked which is the preferable setting ... and you know, I never quite knew either. So it was time then to systematically check this out and see what actually happens at either setting - 1/250 Auto FP and 1/320 Auto FP - for both the Nikon D300s and  D700 ...  Read more inside...