I would say neither. 6D likely won't meet your speed, reach, nor autofocus needs. 70D might not meet your image quality needs.
You might want to wait and see what the 7D MKII brings - apparently being released in next 3-6 months and could bring an improved 24mp APS-C sensor.
If the rumors are correct, it might be the perfect blend of quality and features you are looking for.
Especially with the burst/frame buffer, it is likely the 7D MKII will go unmatched in its price range.
I have a very similar dilemma that I made even more complicated. Last year I destroyed my 60D the 6D was not out yet and I needed a camera. If the 70D was out I would have bought one in a second. But all they announced was the EOS M.
So I when to Best Buy to check out new cameras the T4i Liveview was a disappointment. The Nikon D600 felt like a box that was to big and my fingers cramped it about 5 seconds. (I think I am allergic to the mid to high end Nikon. I was also surprised they had one it was just announced two days before I went to the store.) I was impressed by the Nex. So I grudgingly I bought a New 60D feeling ripped off. I still feel that the 70D is a year Late. If the 6D came out in early September I would have bought one and been broke and happy.
In the mean time Sony put the Nex 5n on sale and I bought one as a Christmas present for my parents to work with my fathers Canon FD Lens. But I just had to play with it first. I liked it so much I kept his FD lens and bough a Nex 6.
A 6D would work for 80% of my photography needs but fail completely on the other 20%. A 70D would satisfy all of my focus needs but fails at IQ 80% of the time. (I am guessing at this but with the NEX my expectations have increased.)
I am going to rent a 70D to see if I am correct in my assumption.
In the mean time download Magic Lantern and have fun.
I also found that for the 80% of photography that I would use a 6D for I could get by with a NEX when it comes to focus.
Now I stuck with the choice of:
- Buy a 6D?
- Buy a 70D?
- Wait for a 7D II?
- Buy a new L Lens? 100-400L or 70-200f2.8L
- Buy yet to be announced full frame Nex?
pick two of the above but I can only afford one.
I really want the Sigma Art 35 but if I stay crop the new zoom would replace the need to buy like 3 primes. I want a Tokina 11-16 f2.8 to replace my iffy water damaged Sigma 10-20. So many Choices so little money.
I still feel tempted to buy a refurbished 6D now that Canon has extended the warranty they are only around $1600 and I have a dead camera to trade in. Regardless I am waiting to see what Sony announces. It would cost to much money to move to Sony for what I want out of my Canon. The Lens are more and not as good. So it looks like I am now stuck using two systems to get what I like.