Sigma serial:1009567 Used with 40D

Please download full res images to compare yourself.

Shot JPG, Faithful PicStyle with parameters to my liking but all shots used the same so they can be directly compared.
Each of the comparison shots is from the best of 3 and no post procesing.

My thought after a few tests (will update)
-Sigma is quite soft wide open at f/6.3 (as expected) but softer than Canon wide at f/5.6
-At f/8 or higher they are much closer in sharpness (at 400mm the Sigma is slightly sharper than my Canon copy).
-Sigma has slight red cast but easily correctable. Canon has more blue cast which is sometimes harder to correct.
-Canon has faster focus but Sigma isn't much slower. (I haven't tested fully yet)
-Tried a bit of AI Servo on the model plane. It can get good shots but not sure about consistency.
-Stabilizer(OS) to me seems about the same as Canon(IS). Of course at 500mm it has to work harder.

New update:
I have found that my Canon is very slightly back focusing and the Sigma is very slightly front focusing.
I'm going to get my 40D serviced to make sure everything is aligned and adjusted properly.
Remember at these focal lengths the Depth of Field is very narrow so even a small miss in focus will result in a blurred shot.