Megapixels are not the relevant issue it the sensor size.
Nikkor DSLR cameras have a smaller than "film" standard sensor either CCD or CMOS and with Nikon the multiplying factor is 1.5x. Canon on the other hand is 1.6x EXCEPT for the Canon DSLR's with a Full Frame CMOS Sensor 36mm x 24mm which is the same as the SLR film focal plane. These cameras are high end and the model's are the 5D, 1Ds MK I, II and III.
The upshot is, if you are going to use a Nikkor 10.5mm Fisheye Lens you are going to need to shoot 6+TB (8 Shots) to cover the 360x180. Where as with the Sigma 8mm Fisheye you can get away with as little as 4 shots in portrait mode using an Agnos MrotatorTCPS , MrotatorUM or UT or 3 shots in Oblique mode using a Agnos *MrotatorC Panohead (*Not compatible with Panoweaver).
My sample shot using the MrotatorC (3 Shot) and Canon 10D 6.3 MP, Sigma 8mm Fisheye can be viewed here
My sample shot using the MrotatorTCPS (3 Shot) and Canon 5D 12.7 MP (Full Frame), Sigma 8mm Fisheye can be viewed here
Just because you "can" shoot a full panorama with the Sigma 8mm Fisheye with as little as 3 or 4 shots doesn't mean you are restricted to this. You may like to shoot with 6+TB also. Understanding the sharpest part of a fisheye lens is the centre area. Though!, if you where to be bothered doing this then you would be better off with the Nikkor 10.5mm Fisheye for it's superior optics.
Don't discount the Tokina 10-17mm Zoom Fisheye as it also is a very good lens and has many more uses outside of panorama shooting.
It really is a well beaten path. most combinations work really well and it comes down to preference on camera brand, cost and what you require. It's hard to beat the convenience of the Sigma 8mm. It is also nice to shoot higher quality with the Nikkor 10.5mm.
This is why I have them all, and my bank account looks so pitiful!