Hi,
I also use a Cannon 50D and the Canon EF 100-400mm f/4.5-5.6 L IS USM lens combination and i couldn't recommend it enough. Wildlife and aircraft images at 400mm have all turned out excellent (unless i have stuffed up as i tried various settings).
The lens is around $2200 at present, which is only $200 more than the Sigma 50-500 lens so is well worth it. You may find some good lens reviews on various web sites that can help. I was leaning towards the 50-500 initially but went with the Cannon lens due to reports that i had read. Unfortunatley, i didn't read many reports on the Sigma 120-400 lens.
They are heavy lenses however and take up a bit of room in the camera bag.
Best of luck in your hunting.
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