Yeah, reminds me, there's a caveat for the Nikon system - the Nikon 70-200 f/2.8 VR II focus breaths, so when you focus on something close at 200mm, the framing becomes something like a 135mm. It doesn't really matter for sports so much as weddings and small events. If that affects your shooting, consider sticking to Canon. Lenswise otherwise, they're neck and neck with a few differences. The sensor really is the largest difference. Nikon has more DR and less accurate WB, while Canon has less DR and more accurate colors (at least in my experience). Nikon's f/1.8 lenses have mostly been updated with a few exceptions and they have a 60mm macro for full frame. Canon has a better 16-35mm f/4 IS/VR, Nikon has a 14-24mm f/2.8G and Canon has an 11-24mm f/4L. Nikon's 24-70mm f/2.8G is metal and heavier, Canon's 24-70mm f/2.8L II is industry grade polycarbonate but sturdier (the Nikon is more likely to break and the rubber wrapping on the focus rings loosen under the same extreme conditions, but they are both quite sturdy). I actually prefer the 24-70mm f/2.8G for its closer focus and manlier lens hood. So really, the biggest difference is probably the new nano-coated f/1.8G primes from Nikon and the 70-200 focus breathing issue.
And the 5D3 is in a different league than the D750 in terms of autofocus coverage. The focus points cover a larger area and have a greater amount of cross type (41). The Nikon D750 has 15 cross type, but they're specced to lower light, and I've heard good things about the low-light AF. The 5D3 AF array, like the 7D and 1DX is black, while I think most of the Nikons focus and display focus continuously in red, so they're easier to see the AF points in pitch black without a seizure from the flashing AF red AF arrays in servo under the 7DII and 1DX.
If you use the AF-on button for back button focus, the 5D3 has one in the right location, whereas the D750, you'll have to reprogram the AF Lock button which is farther from the thumb to replace AF-on. This doesn't affect me too much, as I've recently moved away from back button focus. The top LCD screen might display less information than the 5D3.
Hope that helps your decision further.