Well, I've had 3 of them so here's thoughts on them:
Sigma 120-400 - AF seems pretty snappy and the OS is super. Despite my one issue, which I'll discuss in a second, it would be my favorite of the bunch.
Sigma 150-500 - You would think that it would retain the same apertures as the 120-400 over the same range, but alas, I did not find that to be the case. 6.3 starts somewhere around 350, not 400. OS works well, but doesn't seem to be as good on this lens as the 120-400. Mine had some sharpness issues that were partially rectified by Sigma but still not to my liking. AF is quick but not as quick as the other too due to the longer range.
Canon 100-400 - warmer colors, pretty even in my experience with the 120-400 but a little wider. Biggest personal issue is the push/pull design (some love it, some hate it) and the IS system is older and somewhat useless in my opinion. Based on my experiences with 2 copies, I cannot say it is definitively sharper than the 120-400 nor vice versa.
Here's my issue with the Sigmas - no limiter switch. Sounds trivial, but when you start missing shots because you lose focus for a second and have to wait while it reracks, you'll understand.
I think the build on all three is pretty good quality.
That said, in that range and considering all factors, I would still pick the the 120-400. Colors can be warmed in PP, if you deem it necessary, OS is better, sharpness is a toss up, build is pretty equal, warranty period is longer if bought new and it is cheaper. I can't justify the $400-500 premium for the 100-400.
I would shy away from the 70-200/tc combo as I think you are gonna end up wanting more reach given that it is only slightly longer than what you already have (250mm vs. 280mm).