If you haven't found it, there is a nice comparison chart on their web
page. Go to
On the left look for "Compare Models" and click that. The pop-up has
the useful information on comparing the units.
I've got the 750 because I want to be able to sharpen some very large
bits I have. Don't need it often, but when I do...
Otherwise, I'd recommend the 500. Being able to the two bit angles is
very nice. The 135 cuts like you wouldn't believe. It's so fast you've
got to be careful as you can snap the bit from the torque.
Cutting the split point is a sloppy process. This is the one area the
drilldoctor falls down, badly. It's just a crude push and grind
operation. You manually have to control the depth of the grind and the
results, all from the back end of the bit. It's very easy to
over-grind, and have to restart the whole process again.
They do have a left hand bit chuck that you can purchase separately.
If you have left hand bits, that would be worthwhile.