In exactly the same boat for the last few months but i am leaning towards the sigma c now... reason why is:
I did want the canon mk2 but 400mm although fine for motorsports just doesnt cut the mustard when it comes to wildlife and birds in particular... ive tried the mk1 at silverstone and i can imagine it would be great for airshows as well. Its the sharpest of all the lenses and of course you can crop.. but the sharpness at 400mm between all three lenses would require some pixel peeping to see... itd be a hair sharper but not obvious.
So the first thing you need to to establish is how much length you really need as 400mm may leave you wanting more. I have an old sigma 400mm prime which i used to shoot birds with and even ones 20-30ft away you have to crop in hard.
The sigma has an af firmware update coming tomorrow or today to increase af speed by 20-50 percent.
Reviews seem to converge on the point that the tamron is a tad sharper in the centre of the frame but the sigma is at the edges.
With the sigma c you also get the panning modes, its a bit lighter, zooms and rotates like a canon lens etc.
All good lenses but there are some differences.