Hi Gery,
If you want the 'best' lens, get the nikon. AFS, VR, internal focusing, a 9 blade aperture diaphram (smooth out-of-focus areas) and nikon glass make it the best option by far.
If money is more important, the nikon 105mm VR is quite a bit more $$. I'd look for a used non-vr nikon 105mm, and i'm sure there are plenty available (at the same $$ or less then the sigma or tamron) from people upgrading to the new 105mm VR.
Personally I am snotty about my nikon lenses vs 3rd party. Others love their sigmas, tokinas etc. Every time i try a 3rd party the optical differences are immediately visible.
I think the reason to by a sigma or tamron is either because money is a major issue or if they offer a truely unique lens (like the tokina 10-17mm FE or the sigma 15mm with super close focus.)
Always check your housing manufacturer to see what you need to house each one first. The 105mm VR is fat and i'm not sure if all housings can take it.
hope this helps. Cheers,
Chris