Didnt think Nikon made an 85mm macro? They did an old 55 MF, a newer 60 AF, a new 60afs, a new one for dx (not sure of focal lenght, is that the one you are talking about?), the 105 and 105 vr and the very nice 200mm. the tamoron is a ff lens. I seriously can not fault it and doubt you could notice any difference with a 105 nikon in terms of pic quality. Best bet is go to a camera store with your body and try both, shoot same pics and go home and compare/analyze.

I have seen the tamron 90 advertised in a camera house catalogue for 500 odd, they are about 370-400 on ebay.

cheers