With the Sigma 4.5 I take 4 shots at 90° with a Nikon D3100, which is a DX or APS-C sensor and the image is full circle so John is right when he says you lose a lot of pixels because you are only using about 34% of the sensor.
You could get away with 3 shots round @ 120° intervals, but I prefer the extra overlap with 4 shots round @ 90° so that I can use PTGui's Mask tool to eliminate "ghosts".
Most of the panoramas on http://www.keithprowse.co.uk/virtualtours/index.html
were made with the Nikon D3100 and Sigma 4.5mm lens.
We "cropped" the panoramas because the roof space is usually a lot of pipes and wiring and there are often stains on the carpets, but the lens does give a full spherical panorama and PTGui makes a good job of stitching them.
Ty all for the input.
I suppose from the examples shown that Nikon 10.5 has the advantage in picture quality over Sigma 4.5. Since I am interested in making really quality panoramas I will wait until I buy my first full frame camera (will probably go for the D800) so I can shave my Nikon 10.5, and use it to take 4 pic around.