Not a scientific, but a practical performance comparison between the two full format fisheye lenses.
Conclusion: The winner is the Konica Hexanon lens.
Optical: The Hexanon is a excellent performer even wide open, while the Sigma needs to be stopped down to reach the same edge sharpness.
Mechanical: The Sigma is more bulky with a large curved front lens, while the Hexanon is more compact and so much nicer to handle.
Equipment: Konica T3n, Fuji Reala 100, Manfrotto 190 Tripod, Mirror and Aperture prefire from the Self timer, cable switch.