I have used both Nikon 105 MM VR and Sigma 150mm 2.8 Macro. I think the Sigma gives you the same sharpness but much easier to use than the 105mm.
The Sigma has a tripod foot which you need for sure for micro shot on tripod. Using the 105mm, you could only put the camera body on the tripod - much less stable. Especially if you put on an extension ring to increase your x power.
Sigma gives you more distance to work, at 1/2 the price, has HSM, etc.
105mm could be used as a portrait on FX body and that is the only advantage I could think of.