I am struggling with the same decision. 90% of my shots are indoor sports...with fast moving action and poor lighting.
I have a 40D now...and my go to lens is a Canon 70-200 f2.8 IS II...and my walk around is the Canon 17-55 f2.8. I have a couple of other lenses, but these two are responsible for most of my shots.
A 7D Mk2 would let me keep the 17-55 and give me great sports photography. A 6D would give me the better low-light performance that I crave. I suppose if I had to do it today, I would go for the 7D Mk2...if it existed...and looked like it had decent specs...and the price was not ridiculous. I suppose the 7D's final specs and price will be the final decision maker for me.
Funny we have the same lenses, and the same thought process, although you are more seriously into the sports shooting. I would love to keep with crop (a 7D2) and my 17-55, but between the fact that it must be at least a year or more away, and the fact it probably won't gain more than 1/2 stop in ISO performance, I think I am deciding to go FF.