Imho the only correct answer won't make you happy: it depends on the scene.
Some crop shots are indistinguishable from ff or respond very well do noise reduction (not that there are great new algorithms like DxO's PRIME around), and for me some crop macro shots look even superior to ff because the crop "crisp" look goes along with the subject's texture.
Then again, if shooting gradients crop quickly falls apart after some postprocessing because downsizing cannot restore a smooth color transition, or with skin tones and skin texture every bit of nr smudging given an instant plastic look even at low magnification.
If you are not sure about 6d or 70d, my advise definitely would be 70d because it's the better all-around camera, the 6d specializes and excels in some areas but is crippled in others. Just be sure you know what "low light" means (try to meter the LV with your current gear) because even in cloudy daylight crop shooting can become a constant struggle to decide between lower iso or higher shutter speed, resulting in less keepers - the higher iso capability of ff relieves you of that tradeoff and you can concentrate more on the actual shot.
What's your current gear btw? If on a budget it might make sense to go with a 60d and get a better lens, or if you're looking for good iq Canon crop isn't a good choice at all and you should have a look at Nikon...
Your answer makes a lot of sense - in fact, those are my thoughts exactly. I agree that it depends on the scene, and that 70d is a better all around camera, whereas 6D is crippled in some ways. For me, its better low light and shallower DOF capabilities are not enough to offset the 70D's price, better AF, frame rate, cheaper lenses, etc. Now, if the IQ is substantially better as most posters claim, that would be an important factor. Else, if it's a slight difference that most people won't notice without pixel peeping, then I can live with that.
I can afford either of them, but since I don't make money out of my hobby, I don't want to spend more than I need to. I also want to make sure I am happy with my choice for a few years ahead, because I don't like to resell gear. Speaking of gear, I have too many lenses to make a switch to Nikon, both crop and FF: Tokina 11-16, Canon 17-55 f2.8, Canon 17-40, Canon 24-105, Sigma 30 f1.4, Canon 85 f1.4, Canon 70-200 f4 IS, and a few more :-). Depending on which camera I get, I may sell the crop lenses and get Sigma 35 f1.4, Tamron 24-70 f2.8, or both.