New 8-15 mm vs Used 15 mm Fisheye

Steve Balcombe wrote:

I don't know anything about the sharpness of these lenses, I've never seen a comparison test.

You've ruled out the two most obvious applications for the zoom capability - circular fisheye on FF and diagonal fisheye on APS-H/APS-C. That leaves just one, which is 180deg diagonal fisheye at aspect ratios other than 3:2. For example, you can shoot at 11.7 mm for a 180deg field of view when cropped to square without unnecessary loss of resolution.

If I wanted a fisheye for full frame I would take the zoom without hesitation, simply for its creative possibilities.