I've been wanting one of these for a long time, and finally dropped some cash for the 500X.
I didn't realize until recently that the only difference with the 750 is that there is an adjustment knob that quickens the process if you need to grind a bit twice in a row ie. first attempt didn't work properly, or bit was chipped or damaged requiring additional grinding.
This is really no big deal and saves minimal time.
Also the 500X CAN sharpen bits larger than 1/2 inch with an optional chuck.
Just wanted you to know in case a a 500X became available.