I am so close to buying one of these two lenses, both with image stabilization for my Nikon D7100.
Can anyone help me decide by giving me any important differences or sharing any experience?
Nothing first person, but Photozone.de's tests and reviews are well regarded. Here's the relevant links:
To summarize: Build quality of both is good but not fantastic Neither is a great lens to manual focus Both have poor distortion at 17mm that goes away as focal length increases Resolution is pretty close at wider focal lengths, but at 50mm Sigma is clearly better, especially wide open.
If I didn't have Canon's 17-55/2.8 I would choose the Sigma.
If you want to compare the Tamron SP AF 17-50mm f/2.8 XR Di II LD IF to the Sigma 17-50mm f/2.8 EX DC OS HSM, the image quality comparison is available here.
Roll over the images to see the difference, check for acuteness (perceived sharpness), chromatic aberration, vignetting.
I tried to summarize my opinion, but you should check all the combination of focal length and aperture you are interested in.
Now, if you want to compare the Tamron SP AF 17-50mm f/2.8 XR Di II VC LD IF to the same Sigma, the comparison is available on this link.
This "VC" lens seems to be pretty different if you compare it with the non-VC lens.