Gramercy holdfasts and Lee Valley planing stops etc. are 3/4", so that's what I used.
If you plan to use holdfasts don't apply finish to your dog holes! Dan and I both did this and the holdfasts wouldn't hold fast because of the slickness...This is my first workbench. I chose MDF thinking that I might eventually have to replace it - but that overboring idea might get me a couple more years if it happens. I was going to harden the holes after I bore them with CA or poly.