If you check out both on Flickr groups I think (and you know what they say about opinions) the tamron group looks like it had generally nicer photos, that could be users ability also, but I think generally the image quality looks nicer as opposed to image content
Look at the shutter speed on 99% of the Sigma photos that are currently uploaded. Shutter speed isn't just the first issue... proper focus acquisition, correct hand-holding, proper use of the optical stabilization, proper sharpening, and not cropping in massively... all issues that you end up seeing with beginners who hop on with a new lens.
Flickr is a terrible place to go to judge the performance of a lens.