If budget is not a problem, then definitely the Nikon 
At this point the Nikon and Canon 105/100mm macro lenses are still the top among the list in terms of overall performance, do read the reviews if you are interested 

If you are considering the sigma, try the sigma 150mm f2.8 OS macro lens. It's one of the macro lens that stands out among the crowd for the unusual long focal length (150mm, compared to normal 100/90/60mm macro lenses). The image quality is excellent too.

I am using the Nikon 105mm F2.8 VR both as a macro and portrait lens 
Its a very good lens, and I love it more than my 24-70mm F2.8 haha 

Please do take note that the higher the focal length (e.g. 150mm vs. 60mm) means that you can achieve 1:1 magnification at a farther distance, which may not be a good thing since the farther away you are, the higher the susceptibility to shakes. Can read here if you want to know more: http://pixelsdimension.com/magnification-f...sked-questions/

Happy choosing