I have the 70-200 f2.8L ISII and I recently got the sigma 85f1.4 which I was deciding between getting the canon 85f1.2L
The primary reason for the 85 for me was for wedding use
I decided to go with the sigma over the canon mainly due to focus speed
the canon 85f1.2 is slowwww to focus particularly if you change from a near object to a far object then back to a near object , if changing focus at similar distanced obbjects its not too bad. but the sigma is much snappier I would estimate maybe 3 times faster.
However I thought the canon f1.2 Was sharper at 1.2 than the sigma was at 1.4 the canon is no doubt a stunning lens, if you can live with the slow AF then it is the lens to have for IQ in my opinion it lives up to canons claim of the definitive portrait lens. The other thing I didnt like was the fly by wire manual focus rather than a mechanical manual focus. just not a fan of this setup
I have tested the sigma against the 70-200 and I find the sigma at f2 significantly sharper than the 70-200 at f2.8 and for weddings moving my position i can cover everything the 70-200 can so I have found that the 85 has almost completely replaced my use of this lens for these events. Primarily because it is so much lighter and smaller, less likely to bang the lens into something and shooting for 10 hours the 70-200 weight really starts to take its toll (still a brilliant and all round awesome lens which i love so dont get me wrong here) I find the sigma 85 IQ to be about equal to the 70-200 wide open at f1.4 vs f2.8 but at f2 the sigma is gold.
My copy is heavily front focusing and i had to dial in +13 AF microadjust so while i have a break in wedding shoots I have sent it in to be looked at to see if they can get it closer to neutral I dont have a problem with a little micro adjuct but +13 is significant and I was worried that a future body might not have the adjustment range to compensate.
I dont know if that helps your decision, just some information from my experiences to factor into your considerations the sigma is 2 and a half times cheaper than the canon 85L too