If I were going to a circus and it was ok to take a big SLR and big lens and all that, no fuss, I'd go for the camera with the best ISO performance and ceilling, and either a telephoto with a wide aperture, or a wide lens. I'd want both probably. Telephoto with fast aperture for detail shots of animals, animators, performers, etc. Wide for context and overall capture of everything (even a fun fisheye would be a good time here).
If I were going and didn't want to lug all that, I'd just take a little mirrorless like an EOS-M, with a little wide lens and just enjoy context photos (I'd want an 8mm fisheye with it for example for fun circus context photos of all).
In your case, both SLR's available, I'd take the 6D without a question, for ISO performance. Then figure out your lens situation. Shooting at ISO 12,800 is possible with that camera and they be still very decent and workable, so F4 isn't a limitation for light gathering. Makes lens selection easier.