Hi George,
I've got both, and only use the Nikon 10.5. On my Nikon D80, the output from the Sigma 8mm 3.5 is downright fuzzy, gives a "Hamiltonian" look to panos. I've returned it to Sigma who sent it back saying it was "fine"...
I do use a Sigma for highest end work though, and that's the 10-20mm EX Zoom they have. It's more work to shoot a full 360 though (the only kind I do): 16 pictures (4 + 8 + 4) vs the Nikon's 7 (6 slightly tilded downward to minimize the Nadir hole, plus zenith).