Sigma 150mm F2.8 Macro

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.