It is quite possible to shoot with the Peleng and Sigma 8mm in manual mode on either Canon or Nikon bodies.
Though on a d50 or d70 don't expect the light meter to work with a peleng. Will on all canons though.
The Nikkor 10.5 lens is in fact a full frame [read rectangle]180 degree angle of view fisheye lens from the upper left to the lower right corner of the frame. I think you would take 6 images on the horizon and select the Panoweaver 4 FULL FRAME stitch option.
Where as the Sigma 8mm is a full circle fisheye lens. I think you would take 4 images on the horizon and use the Panoweaver 4 DRUM stitch option.
IF your camera was a full 35mm sensor flavor like a Canon 5d.. then the Sigma 8mm places a full circle on a black rectangle and you would take either 2 or 3 images and use the Panoweaver 3 Pro or Version 4 CIRCLE stitch option.
Sorry to confuse you!
Sigma 8mm = 180 degree circular fisheye image
Nikkor 10.5mm = 180 degree diagonal full frame image
Sigma 8mm will be a lot easier to use and stitch and you can get away with only 4 images to make a full 360 x 180 2:1 aspect ratio panorama image.
The Nikkor 10.5mm will require a minimum 6 images in rotation and one up and one down shot to create a full 360 x 180 2:1 aspect ratio panorama image stitched with Panoweaver 4.0 in full frame mode (6+TB). A lot more work! but because the lens produces a clearer and sharper image you will get a better resolution panorama.
You need to look at panoramas produced with the Sigma 8mm and decide if this quality is good enough for you. (If not your step up is the Nikkor 10.5mm)
Thanks a lot for those answers... As the prices are almost the same (only 50-100EUR of difference), I will go for the 10.5mm lense as it produces better quality panoramas.
I can't wait to show you guys my first panoramas