It's hard to beat the Canon 100-400 L. If I was currently looking for a long telephoto that would probably be top of the list right now.
Do you think the Canon lens and the slightly better F-Stop out weighs the consideration for longer focal distance?
No doubt it's sharper, and you can use it with a 1.4x TC which will get you the extra reach if needed. So yes.
But would not the 1.4x cause reduced light into the camera, therefore taking the F-Stop value to similar to that of the Tamron, but the Tamron has a longer focal distance so does this make the Tamron a better choice? So now it is saying to me, the Tamron can capture images at the longer distance, but would the quality of the Canon 100-400 at 400mm be better than the Tamron 150-600, at 600mm, when cropped? I hope that makes sense.
I think what I am trying to say is, a shot taken with both lens, at max distance, when cropped could the Canon be a better image (without a 1.4x).