I've now had a chance to have a quick play with a 120-300 OS alongside a 300 f2.8 IS and wasn't too shocked that the Canon came out on top. S a quick comparison in case others were wondering...
The Sigma does feel well built and the plastic lens hood feels a lot more solid than I had expected. Both the focus ring and the zoom ring were smooth with a nice level of resistance and the lens felt good for handholding. The OS seems to be quite effective as I was able to handhold it at 300mm 1/50th and still get good results. The Canon looks like a bigger lens though there is little difference in size and the Sigma is slightly heavier. The Canon is also very well made and feels great in the hand, the IS is also very effective. One thing that was instantly obvious when testing them was the fact that the Sigma comes up well short of 300mm when shooting at close range (a common feature of zooms but I was surprised at now noticeable it was). For me this is a big issue as I will be using the lens for wildlife and the shorter focal length would be exaggerated when using with tcs. Then on to the real test how sharp they are at f2.8. I was very impressed by the Sigma, on a very basic test shot it showed good sharpness and contrast. It was significantly better than even he best non-OS version that I have used. Again as expected the Canon was even better, sharper better contrast and colour rendition. This was a mkI Canon so sells for about twice what the Signa does but probably isn't twice the lens. The bottom line for me is that as expected the Canon wins out in just about every area, only lacking in the versatility of the Sigma. However the Sima preformed better than I expected and I reckon I would be very happy to shoot with one, but there is no doubt in my mind that the Canon is worth the extra if funds allow.
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