Actually, Full Frame is not what people always aim to reach. Some go for FF because of need, some go for because of temptation of wants.
Even if it's forced to choose between 6D or 5D3, it's still based on your needs and budget, both are good camera if you refer to image sensor. But if you compare the AF that 70D uses (I heard this uses same AF used in 5D3), I don't think 6D is comparable (at least from the specs sheet. I've not used the 6D before.)
I guess i fall to the latter category haha
so my point is, 5d3 combines both the decent AF of the 70D with the FF of 6D. So i would be keeping my 70D for sports purposes, did perform good enough for me haha