@gardna - I shoot a lot indoors where lighting condition are poorer so the 6D and D750 which I used to use were very helpful given their good high ISO performance. I havent used the 5D3 so I cant comment. Again, it depends on your needs. Try not to pay for features that you wont actually use, or would only use less than 3-5 times during the camera's lifespan.
My guess is you just want to get a FF because you want a FF. If that is so.
Then go ahead. 6D/5D3 are FF. You decide whether you want 2 body (keep your 70D) or 1 body will do for you.
I say.. keep the 70D first. Who knows..you might not like the FF after you gotten it.
This is one of the reasons why i wish to go for the two body set up of 6d and 70D
i dont think ill actually use the extra features (of the 5d3) that much i guess
But ill survive with my 10-18 for now i guess given the hefty, hefty price tag of the 11-24
i guess i might not get used to the crop factor of FF given ive used a crop for a long time