Full frame certainly is a step up, in many respects. I went from a 60D to a 6D, also. It took me about a year to replace all my ef-s lenses with EF. Then I thought I missed the reach of APS-c, and AI-Servo on the 6D is mediocre, at best. So, I got a 70D. For most shots, it didn't matter if I used 70D or 6D: long work on the 70D and thin DOF on the 6D. Everything else didn't matter. Then I briefly got into video, and picked up ef-s 10-18, 18-55, and 55-250. After a few months I decided I really suck at video and sold all the aps-c stuff, AGAIN!
Then I tried using the 6D for sports/action stuff. I was not sure if the problem was me, or the 6D, but I was not satisfied. I bought a 1DIII about 6 weeks ago. That provided proof that it was not me; the 6D sucks at AI-Servo. I have shot two athletic events with my new 1DIII, totaling about 800 shots, plus some chimping along the way. Of the 800+ shots I brought home, I had exactly four (4!) OOF shots. Two were certainly my fault. I'll take the blame for the other two also. I am liking the results I am getting with the 1DIII so much that I bought a 1DIV. In order to get 1DIV performance out of a full frame body, I'd have to go to a 1DX (II?), which I can NOT afford. I hope I like the 1DIV as much, and it works as well as my 1DIII does. It arrives in about 4 hours.
I really do enjoy my full frame 6D, but it can't do what a 1DIV does. Full frame isn't everything, to everyone.
SIDEBAR:
I briefly considered the 7DII before buying the 1DIV. The AF issues, while few and far between, are still being reported. While making no comment about the 7DII because I've never shot one, the 1DIV is proven technology, and the AF is a LOT easier to configure (yes, I read manuals). That works for me. If I were focal-length challenged and needed 1.6X reach, I would have gotten a 7DII. Seems the vast majority of 7DII owners are happy with their camera.