Actually, I am using a Sigma 105mm sometime to take portriat too, but it seems like I might be having a very good copy as my bokeh isn't as bad as what others had perceived. But if you have the means to get a Nikkor 105mm, I don't see the reason why you should not go for it .... I for one will go for the Minolta 100mm Macro lens if I have the money for that minor extra mile of quality (not to say Sigma is no good, but the extra mile will be better) by Minolta, Since I am using a Minolta body primarily.
i am just checking reviews. in general people seems to like or rate
the tamron as #1 then nikon then sigma. As color renderatin and image quality. It does have shorter working distance then the sigma but that also applies for insects and movign objects.