If the bottom is THAT hard you probably won''t get any fluke-style anchor to dig in. The difference in weight between a Fortress and a Danforth won''t matter. Besides, in my experience here in Chesapeake mud, the design of the Fortress, which allows you to increase the angle of the flukes for mud, is much better at digging in than a Danforth. My back-up anchor is a Danforth Hi-Tensile, and I have had much more trouble setting it than I''ve ever had with the Fortress.
For a hard bottom, wouldn''t a CQR or Delta be the anchor of choice anyway?