The 6D will be better and less grain at higher iso.. but... speaking practically - you can correctly expose images at 10,200 and 16,000 iso with the 7D2 and still have a picture. Not a picture you want to blow up to 20x30 but still have one for 4x6 just fine to go in that dance or sport album. I have a 5D2 (like 6D AF) and a 7D2 and between the two, I'd keep the 7D2 if I had to sell one. the AF is so much better, and the difference in IQ while there, is not as great as the crop vs full frame once was.
I don't think either is a wrong choice, just there are trade-offs and adjustments to whichever one you choose.