I just looked at the comparisons between the Sigma and Canon 1.4 on The Digital Picture's site. From what I can see, the Sigma destroys just about all of them in IQ. 50 is probably my favorite focal length. I believe I can see this lens in my future.
I just got the 135 used, but haven't had a chance to get out much with it. The few photos I got are pretty nice, but I had to do a bit of focus adjustment in the menu. I love the 50mm focal length, but I have the 50 1.4. It's kinda meh wider than 2.8. I will probably get the Sigma if it lives up to the hype. I think you would probably get more use out of the 50. It's perfect on the 6D. From what I've seen so far, the 135 has a special quality that I love, I just won't get as much use out of that focal length.
The 6D is a great low light camera, but at low light, it becomes a 1 point auto focus camera. The outer points are basically useless for low light or contrast areas. I figured it would be this way when I looked over the specs and have adapted to the cameras limitations. I agree that the 5d3 would better suit you. I'd love to own one, but I guess I really don't need the AF for what I'm shooting (would love to have it though )