I have not seen any review here for the Tamron 100-400 4.5-6.3 so here it comes
I have used the Sigma 100-400 F 5-6.3 for about a year and was never really happy with it. The focus and IS were not great but the IQ was great. Also it does not work with extenders, it is listed on the Sigma site for use in manual focus only with a 1.4 or 2.0 not to mention no tripod mount The weather resistance seal is at only at the mount which from what Sigma says is 75% percent of their problems with water/dust.
I was intrigued by the early reviews of the Tamron 100-400 so took a chance and ordered one from B&H. In the side by side comparison the Tamron was a hair sharper at 40 ft at all FL's, but I would think well within production tolerance. You had to be at 100% to see any difference. I did find the focus faster and more consistent with the Tamron and the VC better than the Sigma OS. At 67 I could get 3 stops with the Tamron and 2 with the Sigma.
The two things that were a major advantage to the Tamron for ME were weather resistance and the Tamron worked great on my M5 as well as my 7D MII. Sigma shows the 100-400 C sealed at the mount but Tamron shows their 100-400 has 10 seals from the mount to the front element.
Dustin Abbott has a good final review you may want to watch
So now the Sigma is on consignment.