Here is a very interesting thorough comparison of these 2 cameras
As said before these 2 cameras are very good, never the less the 5D III is pretty odl while the D750 enjoys newer technology, the rumors say the 5D III should be replaced soon.
Dynamic range and low light the D750 has the edge.
AF of both cameras are very good, I saw contradicting info, some give the advantage of the D750 and some to the 5D III so I would choose either one due to their AF because they are both top notch!!!
I own the D750 and its an outstanding camera, really amazing and the fact its cheaper then the 5D III just makes it ever sweeter.
Canon has 2 24-70mm 2.8, the old version was very good but Nikon's 24-70mm 2.8G was consider better, the new Canon 24-70mm 2.8 is better then the old one but I believe its also much more expensive.
Under no circumstances would I go with a superzoom lens like 28-300mm you asked about or ANY other superzoom lens no matter who makes it as ALL superzoom lenses offer great focal flexibility but substantially inferior image quality especially compare to the pro 2.8 zoom glass.
Lens will always be the camera bottleneck and it makes no sense to buy excellent FF camera and put a mediocre soft glass in front of the sensor! well thats how I see it anyways
My advice get the D750 but either camera will give you many years of happiness and great images!