ok, i've had the sigma for a while now and have recently had the pleasure of using the 105VR which i subsequently bought. There have been alot of questions about macro lens' in here and i've always recommended the sigma so i thought i'd give people a rundown of the tests i've been doing in numerous side by side comparisons so i can give people a better idea on the performance of these 2 lens' for macro work.

i'm not going to post any pics cos there's just too many of them, and quite frankly, i'm too lazy, so you'll just have to take my word on it. lol

AF speed: Nikkor wins by miles.
it's actually a very fast AF lens which surprised the hell outta me. its faster than the 50mm f1.8 which is pretty darn quick. thats amazing really considering all the glass in there and the fact that its a macro lens. especially if you manually focus first to get roundabout the point then use the AF, it locks on instantly. so this is can double as an ideal portrait lens while the sigma is just too cumbersome to use for that. the sigma takes a while, frustratingly so at some points. it feels sluggish and downright pedestrian by comparison.

AF accuracy: Nikkor hands down.
it just didnt miss a beat. even really low contrast items, it locks on almost every time. the sigma worked much better on my d300 than my d80 but still, not in the same league. this is particualrly evident using the extreme AF points. on say 50% of occasions in dim light or low contrast the sigma will hunt first.

Sharpness at small apertures: Even
couldn't pick any difference even at 100%. they are both amazingly sharp. however, if you are planning on using the 1.7tc then expect to lose a significant amount of sharpness. much more so than when i use the TC with the 70-200VR.

Sharpness at large apertures: Nikkor by a whisker
very marginal differences when viewed at 100% at f2.8, f4 and f5.6. but normal viewing, even 50% crops, no differences evident at all. TC result as above.

Contrast: Sigma by a country mile
one thing i've always loved about the sigma is its awesome contrast. it resoundingly smacks the nikkor's butt here. at every aperture the sigma is better and more vibrant. this is especially evident in crops, but even in full size, the sigma is noticeably more contrasty which personally, i like. this is really frustrating for me cos it means i will have to add astep in my PP with the nikkor where i never had to touch it with the sigma.

Colour: Even
the sigma's colours always appear more saturated and vivid, mainly due to the contrast i suspect. the nikkor's were neutral every time and more accurate. depending on the shot, one lens will outperform the other. for shots where the colour saturation is important the sigma is the winner. for shots where the colour accuracy is more improtant then the nikkor is nicer. an important thing to note here is that with the nikkor you can bump up your sauration and retain alot of detail in the colour, the sigmas contrasty nature sometimes removed all the detail in bold colours which you cant ever get back.

Lens Handling: Nikkor by miles
its just beautiful to use. MF ring is huge, its light enuf to handhold comfrotabley. it fits in the hand nicely and has perfect balance. the sigma has always felt a little clunky to use and i've never been really that comfortable shooting with it handheld. and of course, the nikkor has VR so its handholdability is increased again cos the VR works perfectly allowing you to get shots at insane shutter speeds that are still sharp. this is really useful, much more so than i expected...even with flash.

Build Quality: Nikkor by a decent margin
both very well built lens' but the nikon has the edge in finish and it just feels like it can never bust. the lens hood is also superb on the nikkor. it's just about as nice as the 70-200 and thats really saying something.

Manual Focus: Nikkor by a whisker.
the damping of the MF ring is just right making it easy to MF with one finger. the position of the MF ring, (due to the size of the lens i suspect), is right where you need it aswell. the sigma is a bit harder, is damped a little too much and you need to move your hands to MF just a tad which can make or break a shot.

Focal Length for bugs: Sigma by a good margin.
bit of a no-brainer. you can get much closer and the success rate is much higher for the sigma cos you dont need to be so close to fill the frame and get the shot.

Focal length for other stationary objects: Nikkor comfortabley.
again, a no brainer cos its lighter, smaller and more comfortable to use making flower shots etc easier and less taxing.

Bokeh: Sigma by a small margin
cos of its longer focal length you can get the nicer dof and can isolate particular items better by removing distracting backgrounds much easier and more effectively. the bokeh is just beautiful and creamy on the sigma, the nikkor is no slouch and given the right circusmtances its just as good but shot for shot, the sigmas bokeh was always just a nose in front.

so there you go, thats my review of both of them. i bought the nikkor cos i want 2 macro lens'. i want one for stationary stuff and its smaller size and weight will let me take it in my kit far more often. but for bugs i want a longer focal length. so i will be getting the nikkor 300mm f4 afs and using it with extension tubes for my bug shots and then selling the sigma.

hope you get something out of the review and it helps in any decisions you make. its hard to pick a winner here really, the nikor is certianly a better lens all round and wins in almost every respect; but its lack of contrast in comparison to the sigma and shorter focal length lets it down if its your only macro lens and you want to shoot bugs.