chief,
I have both currently and have had several of both in the past. There is little to no difference between them. IQ wise that is. The Sigma dock is a plus but a small one. The Tamron is better sealed than the Siggy C, a small plus, I suppose. It truly is a, your choice, proposition. Pick one, you can't go wrong either way.
Now if you consider the Sigma 150-600mm f/5-6.3 DG OS HSM Sports Lens for Canon, it is a horse of a different color all together. The IQ is slightly better than either of the cheaper versions from Tamron or Sigma. It is built as well as any Canon L lens. It can use the Sigma dock. It is big. It is heavy. It is fantastic!
I don't have a 6D so I can not tell you they work on a 6D but I can see no reason why they would not. They all work beautifully on all six of my Canon bodies of differing models.
EB
EOS 1DX and many lenses.
