The Sigma is quite a bit longer in focal length, and will give you the same sized image from about a 50% farther working distance than the 105 would. I have seen quite a few photos done with the Sigma 150,and it has a very beautiful pictorial quality, as does the new 105mm VR-G Nikkor.
Not sure if it's the Nikon 105mm is the VR, or the older AF-D model you are considering. I have a two tele-macros: a 90mm Tamron and a 180mm Sigma EX macro. I find the 180mm is my preferred lens. I think the 150mm lens is larger than either the Nikon or Sigma 105mm lenses, but a longer lens is usually bigger and heavier,and well, longer.
If you want to shoot photos of small insects, butterflies, etc, I'd say the 150mm would be the easier choice to work with in terms of getting photos with a nice,selective angle of view and good working distance that will allow you to use flash easily. All three of your choices are known to be excellent macro lenses.