I was just looking at this same comparison a couple of weeks ago. I ruled out the 100-400mk2 because of cost. However, I was comparing the Tamron with the 100-400mk1, which can be found for pretty good prices right now as dealers are trying to clear them out (especially if you combine that with the rebate expiring tomorrow). From reading reviews and countless forums, here's what I gathered:
Image Quality: A toss-up depending on focal range. At some focal lengths, the Tamron is sharper, at others the Canon excels. The 100-400 with my 1.4x teleconverter gets me out to 560mm too (at similar image quality as the Tamron at 600mm), but will only auto focus on the 1D series, 5D3, and 7D2.
IS: Tamron probably a little better.
Focus speed: Canon probably a little better.
Reliability: Tamron has a much better warranty, but the Canon is pretty much guaranteed to work with every EF camera during its lifetime. Read some reports of Tamron's AF not working on some modern camera bodies, even with the new firmware, so I wasn't sure what to make of that.
Ergonomics: Canon is much smaller and lighter to carry around… Tamron wouldn't require me to swap off the teleconverter if I wanted just a little more zoom. I've tried the push/pull 100-400, and don't mind it at all.
I ended up pulling the trigger on a 100-400mk1 and love it so far.