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 1D Mk IV and less lenses then before!
