Let me give you my thoughts. i owned the 100-400 for a few years, not thrilled w/IQ but rented other copies and it was fine for my purposes. I've had the Tammy 10 days, done limited shooting due to bad weather etc.
As far as IQ, focusing speed, handling etc, I find no big difference.I've read a lot of threads arguing one way or the other and complaints about the focusing of the Tammy in AI Servo. I'm not going out on a limb and saying the Tammy is "better"...just a toss-up. let's say price is the same, eg, if you thought the Canon was much better, you'd spend the extra $$.
So the question is: What are you using the lens for?? Can you use the extra reach from 400 to 600mm?? I've been shooting mostly wading birds and waterfowl. Even that can be frustrating if I'm home up north and the birds are far away. Attached is a shot of a Red Breasted Merganser I took yesterday. Tamron 150-600 @ f8.0 600mm. These are shy ducks and I never could have gotten this shot(or dozens like it) w/the 100-400L. (BTW, I don't use converters on zoom lenses.)
Now, one neg. for me is weight. The Tamron is heavy for me. But even though it's 1/3 heavier than the Canon, it doesn't feel that much different...I can't really handhold either and need to use a monopod. (This shot was from the car window, leaning on the edge, VC "on")
So I guess you know I'm happy w/my choice. Good luck.
Tamron 150-600 600mm F 8.0 1/2000 ISO 800 VC On sharpened, clarity adjusted in LR
Nice shot, glad to hear it's working out for you.
Roger










