I'm not sure it is fair to compare these to lenses in a competition style fight.
But, because these are still the most currently used lenses for panoramic work I'll happily talk about them.
First of the Sigma 8mm is a circular fisheye with a much wider FOV over the Nikkor 10.5mm full frame diagonal fisheye. Panoweaver looks at these images quite differently with the Sigma 8mm classed as "Drum" style and the Nikkor 10.5mm as 'Full Frame" style.
The Sigma 8mm fitted to a APS-C/DX small size sensor camera requires only 4 shots to cover 360x180 sphere. Where as the Nikkor 10.5mm requires (6+TB) or at a push (5+TB) reducing the overlap to very fine tolerances and creating more trouble than it's worth.
It is true the Sigma 8mm suffers from maybe a little more Chromatic Aberration but this is mostly because it is a wider lens in the first place. Either way, it is an easy fix during RAW processing. The Nikkor at f/2.8 is a slightly brighter lens but again this really isn't important when it comes to panorama shooting because you simply will not be shooting near this f/stop.
Both of these lenses are perfectly suitable for panorama shooting. The Nikkor is the superior lens by virtue of it requiring more images to complete the full 360x180 and thus the Sigma 8mm is a trade off of resolution vs the convenience of less required shots.
Remember we are talking 4 shots (Sigma) vs 8 shots (Nikkor) on the APS-C/DX small sensor camera bodies.
The Sigma 8mm panorama will have a maximum size of around 8000x4000 and the Nikkor 10.5mm around 11000x5500.
If you really wanted to compare lenses like for like then the Nikkor 10.5mm vs the Sigma 10mm vs the Tokina 10-17mm Zoom fisheye @10mm would be a better comparable match up as all are full frame fisheye's.
Possibly if money is the final deciding factor the Tokina 10-17mm Zoom Fisheye would be the clear choice as it is an excellent lens and far most useful because of the Zoom.
Oh! and of course we now have the Korean made Samyang 8mm Full Frame to choose from also!