I am surprise to see this post here (MacRumors). I am a regular at this forum, but for Mac related stuff and not Camera. I am going through the same dilemma - 70D or 6D, and was quite shocked to find the best discussion online is here on MacRumors. 
I want to add in my 2 cents having done a week of so research on this very question.
As a few people already mentioned, the 6D is a full frame sensor and the 70D is crop sensor. That's the biggest different between the 2 cameras. So the question now becomes do you want a full frame or a crop sensor.
This question was really easy for me, I have 2 segma DC lenses. I got the 10-20mm f/4 and 18-70mm f/2.8-4. Although they are considered cheap lenses, they are bloody expensive already. I guess I consider kit lens cheap, but anything above that is expensive.
Both of them will not work on the full frame camera at all (correct me if I am wrong). Getting the 6D is therefore simply not an option - unless I will the lottery tomorrow.
If you don't have any lens already and has the option to go with 6D or 70D, the question is how much money are you will to spend down the road. For quality the 6D will win but that's because you are spending more money.
The 70D actually have more bell and whistle, like the touch screen and flash. And better video recording capability. If you are not a Pro go with the 70D. Unless you are a Pro doing some professional work, you won't be able to tell the quality difference between these 2 good camera to start off with.
Not sure where you are located but the 70D body-only just dropped to $1150 Canadian dollar. It's much cheaper than the 6D. I am waiting for an even more attractive offer (like free memory card or free 1000 photo development) then I will pick the 70D up
This is a good reference - http://snapsort.com/compare/Canon-EOS-6D-vs-Canon-EOS-70D
I want to add in my 2 cents having done a week of so research on this very question.
As a few people already mentioned, the 6D is a full frame sensor and the 70D is crop sensor. That's the biggest different between the 2 cameras. So the question now becomes do you want a full frame or a crop sensor.
This question was really easy for me, I have 2 segma DC lenses. I got the 10-20mm f/4 and 18-70mm f/2.8-4. Although they are considered cheap lenses, they are bloody expensive already. I guess I consider kit lens cheap, but anything above that is expensive.
Both of them will not work on the full frame camera at all (correct me if I am wrong). Getting the 6D is therefore simply not an option - unless I will the lottery tomorrow.
If you don't have any lens already and has the option to go with 6D or 70D, the question is how much money are you will to spend down the road. For quality the 6D will win but that's because you are spending more money.
The 70D actually have more bell and whistle, like the touch screen and flash. And better video recording capability. If you are not a Pro go with the 70D. Unless you are a Pro doing some professional work, you won't be able to tell the quality difference between these 2 good camera to start off with.
Not sure where you are located but the 70D body-only just dropped to $1150 Canadian dollar. It's much cheaper than the 6D. I am waiting for an even more attractive offer (like free memory card or free 1000 photo development) then I will pick the 70D up
This is a good reference - http://snapsort.com/compare/Canon-EOS-6D-vs-Canon-EOS-70D