Hi, i think each lens has its own characteristics. Yes, Nikon 70-200mm is sharper edge to edge, but it does not means that Sigma is not sharp. Sigma strategy is to focus on the center sharpness. I find it appealing especially for portraits where you want to blur the background. But if you're looking for ultimate sharpness edge to edge, you need Nikons.
I use Sigma 70-200mm extensively in full frame FX D700 cameras, I think it performs very well with the camera. The HSM works very well to track down the objects. Check out my dance gallery shot by Sigma 70-200mm.
Lack of VR in this case does not matter, since I shot in high shutter speed to eliminate motion blur.
For crop sensor camera, the focal length is longer due to 1.5 crop factor. It will be a bit challenging to combat shake in 135-200mm, but again, it will depends on what r u shooting. If you are shooting still image, VR maybe more important. You might also check out 50-135mm like other suggest, but make sure you understand that the 50-135mm is not designed for full frame (FX) camera.
I bought Sigma because of its value. I think that $1000 for VR and extra punch of sharpness is too much and unnecessary for my shooting style.
The Sigma's sharpness is good enough for me, more sharper than that will make the image look unnatural.