I use two Sigma lens for about 2/3 rds of my work. Sigma 70-200 f2.8 and Sigma 18-125 F3.5-f6.3. Both work like champs. I've not had a problem with low light focusing with either of mine. But I did use the Canon 70-200 F2.8 L lens once and it did seem to seek quite a bit in low light. Whatever you buy, be sure to test it thoroughly after you get it to be sure its right on in focus, and speed, etc. Many lens of all brands can be basically bad out of the box, so make sure you do your testing during the exchange period. Don't buy from anyone unless you do have at least a 30 day exchange window.
Here are a few photos with the lens above:
Kennedy at the Virgin Islands shot with the Sigma 70-200 F2.8
Haley with the Sigma 18-125.
About half the photos in my portfolio are shot with the above two lens.
Cheers,
rfs