The good news is that by all accounts there's virtually no such thing as a bad macro lens. They are all specialist low volume lenses, rather than consumer grade tat, so all are optically top notch. If you're going bug hunting you'll want at least 90mm.
Here's a review of the two prime candidates by one of the heavy hitters in the Nikon community (he writes the definite ebooks on the Nikon DLSRs).
http://www.bythom.com/105AFSlens.htm
Sounds like if budget is a factor, the Tamron is a no brainer.
I went for the Nikon 105VR, and I have to admit that it's an amazing bit of kit. As stated in the review, the VR is of limited use at 1:1, as it only stabilises you in the X-Y plane, and the Z axis is still all over the place. When you're looking at a depth of field of a fraction of a mm, this is not so good. You've also got the problem that bees etc laugh at your puny low shutter speed that VR permits. At less ambitious distances though it's superb, and I could well believe the quoted 4 stop advantage.
If you're serious about macro work, lighting is critical. For a start a 105VR+hood casts a huge shadow if you're foolish enough to use the built in flash, and I'd imagine the Tamron will be the same. If you're doing studio/lightbox work, you'd be able to make to with improv lighting and reflectors, but if you're bug hunting handheld in the field, some proper macro lighting would seem sensible. Have you looked at the funky new Nikon SB-R1? It's on my christmas list