Well, the SIGMA is a much more flexible tool, and yet a lot heavier.
The problem with the 400mm is it's gets a little to specialized. I loved it, but it really had only one job for me.
When the 100-400mm MkII came along, it did everything the 400mm did as well or better. So that was what "replaced" the 400mm for me.
I like the range of the 150-600mm,. it means one lens carry vs. packing a few options.
I have not shot the tamron, but I understand the AF on the SIGMA is better, the OS is definitely better, and the IQ is a notch up as well.

. Here are a few from today. Both shot handheld.









