Correct me if I am wrong but it is all pure speculation this moment in time.
It is conceivable that it is worse, we just don't know yet.
But for £800, released 10 years after the Canon, it SHOULD be better.
I say wait until it is out, then decide.
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Personally I would get the Canon the price of £800 is street price (it won't be), vs Second hand 35L. Performance wise won't be that different even if the Sigma is better. Then there is the depreciation value. When the Sigma street price drops from the £800, the secondhand price falls drastically with it. Then you have the CPS program if you qualify, 3 day turn around for me to fix my lens. Got it back the same week, can't fault the support you get with Canon gear (should you qualify).
I thought about the Sigma 85/1.4 too, I know it is a good performer but I wanted that 1.2, but losing that focus speed.