It's in the eye of the beholder when it comes to noise, you can analyse it scientifically but it's what you see that counts. My Kingfisher photos were shot at ISO 2000 (due to bad lighting conditions), however I printed one of the images at A3 size and the noise doesn't look that bad. For me judging ISO requires prints and a magnifying glass. I tend to think a lot of people get hung up on specifications and don't get on with shooting pictures and printing them.
As with any Camera the choice always boils down to Ergonomics and a control layout you can get on with. Then you look at other factors like ISO, sensors, AF etc.
As for Sigma's super telephoto lenses, you only need to be strong enough to carry them in a bag (I'm considering a roller type bag to save my poor neck and lashing a tripod to a little stack truck thing). Once you put them on a tripod or monopod the weight issue disappears mostly.
Do you think the 5d3 noise looks cleaner or more appealing than the D750 in the comparison posted? Not everyone prints these days.
Agreed re controls, it is pretty important.