The EdgeHD scopes use a sub aperture corrector placed inside the baffle to eliminate coma and flatten the field.
This means that when imaging with modery large CCD chips, the field will be very sharp all the way across, Vs the blurred stars that would show up in the standard C11 at the edges of the field.
On axis, they will perform the same, but off axis, the EdgeHD is a far better performer. Some say that it only matters for imaging, but my experience was that when using visually with modern ultra-wide field eyepieces, the edges of the field would be comatic and out of focus with the standard SCT, and razor sharp using the same eyepiece in the EdgeHD.
Now people will say that it is not worth the cost, but that is an individual judgment call. If you use refractors and love the wide, coma free fields, then you might place far more value on the EdgeHD design.. I know I did. My EdgeHD 8" as the most satisfying SCT (pinpoint across the field) that I have ever owned. Equal in off axis sharpness to refractors.