Canon 7D Mark II pixel pitch
4.1 µm.
Canon 6D pixel pitch 6.5 µm, or 58% larger.
http://www.astropix.com/HTML/I_ASTROP/COMPARE.HTM
I am only a beginner to astro-photogoraphy, but it seems logical that for the same sized image projected onto the two sensors, the 6D will have lower resolution but 58% more photons captured per pixel. As for the noise, that depends on sensor quality, reading the signal from the sensor, and random noise introduced by the electronics. The 7D Mark II is newer and includes the Dual DIGIC 6 processor which I would be expected to have superior noise suppression. That said, 58% is a significant difference in raw signal strength per pixel.
It would be really nice to see a side-by-side comparison of several astro images...and then wait for the 6D Mk II
