Big difference, not
http://www.dxomark.com/Cameras/Compare/Side-by-side/Nikon-D750-versus-Nikon-D810___975_963
The D750 is better above 3200 ISO you say
And how many pictures do one normaly take at souch high ISOs?
The IQ is better with the D810/D800
With the D750 you get a cheaper ( in some senses not only the price of the camera) a faster and lighter camera
I wouldn't mind having a D750 but it would be as a backup to my D800
I've shot with the D800 for a couple of weeks and recently held the D750 and D810 side by side. Other than size and weight, the two cameras look and feel near identical in build quality.
Now onto ISO... personally I use ISO 3200 and above frequently. If you shoot in perfect light at all times, you certainly don't need a low light monster. If you take your camera out at night or shoot indoors under dim lighting, then you'll have to use those higher ISOs frequently.
Is the D750 leaps better in low light than the D800? No. Is it a visible difference? Yes. Is the D750 better at focusing in low light? Yes. Is the D800 still a great camera? Yes. Would I choose it for low light shooting? No.
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