I can say this much about sigma. Their pro glass, is on the upper end for sure. I have owned a half dozen "pro" level sigma lenses and still have a couple of them including the 120-300 2.8 hsm. Here are my thoughts after many many tries to get sigma to work. . . 1. quality control is lacking, I've seen so many QC issues it's a wonder they can stay in business. 2. build quality is not nearly as good as nikon or canon. In other words, while they do have good glass, you'll notice the build quality - they DO NOT last under active and continued use. 3. color shift, sigma always gave me a green cast as compared to my nikon lenses. 4. customer service is a joke. They almost always gave me the run around and never did the job right. In one case i sent a lens back (100-300 f4 hsm) cuz it was not sharp at 4f (which is the only reason to own f4 lenses is so that you can shoot them wide open). Sigma told me they could "calibrate" the lens and the body - which as is turns out, they cant - but the issue was that I got the lens back from them - they said they repaired it when really all they did was take my serial number off my lens and put it on a different one and sent it back - it was so obvious. as it turned out, the 100-300 is not up to my standards at f4 so i sold it to a press photographer. . . In any event, the point is this, IQ is only one aspect of a lens . . .