I don't know why the OP thinks the Sigma is only $150 less than the Nikon. At B&H, here are some (current) prices:
Sigma 105mm F/2.8 Macro $399
Tamron 90mm F/2.8 Macro $489 (and there's a $90 rebate right now)
Nikon 105mm F/2.8 VR Macro $739
Nikon 60mm F/2.8 Macro $400
Sigma 150mm F/2.8 Macro $599
Don't forget that Macro lenses make excellent fixed focal-length lenses. Pretty much, you can't go wrong with any Macro lens as they all give excellent results. Manufacturers really work at getting their Macro's right.
In 35mm camera terms, 70 to 120mm (generally) was used for portrait photography. the Nikon 60mm Macro is a nice lens and becomes the equivalent of a 90mm lens in the old 35mm terms, smack in the middle of the old portrait lens range.
Tamron's 90mm Macro is an excellent, mid-priced lens with a great reputation. I've used this lens and it handles very well and gives great sharpness, contrast and color. It is effectively a 135mm lens so just a bit over the normal portrait lens range.
The Sigma 105mm lens is again, an excellent lens and less expensive than the Tamron, but doesn't have as large a following or quite as good of a reputation. In 35mm terms it becomes 157mm so is well outside the "normal" portrait lens range.
The Nikon 105mm VR is a stellar lens, is AFS and includes built-in stabilization. Tops in Sharpness, color, contrast but by far the most expensive of the 90-105mm range macro lenses.
The Sigma 150mm Macro is a great lens that I'm currently using. It gives me more than double the working distance (distance between end of lens and object being photographed) of the 90-105mm range lenses, which is why I switched from the Tamron 90.
The 50-60mm lenses give excellent "normal" portrait range performace. The 90-105mm lenses give excellent head-and-shoulder portrait range performace and double as fast, short-range tele's. The Sigma 150mm lens can give great head-shot portraits and double (triple?) as a fast, mid-range tele for sporting events.
You really can't go wrong with any of the macro lenses for macro shots. What else you plan on shooting should help you decide on whether to get the 50-60mm lens, 90-105mm range lens or 150-200mm range lens.
Here are a few Macro shots, all taken with either the Tamron 90mm or the Sigma 150mm Macro:
http://www.pbase.com/mothman13/macros
You can also go here to see Field of View data for varying distances for varying mm focal lengths:
http://www.pbase.com/mothman13/image/74511515
Happy shooting,
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Mothman13
http://www.pbase.com/mothman13