Hmm Brendan...what do you say about this...both lenses wide open and you can also put the tamron to f4 if you want to make it really honest.
Try both the wide and long end.
Link to Bryans ISOchart
Anyway I would buy a Canon f4L over a Tamron or Tokina, but just because I had one and I liked it a lot... it rocked! But for my eyes the Tamron seems just as sharp and even a little sharper than the Canon...(or perhaps I don't get the charts)
And the Canon-AF makes this lens much more versatile and usable...at least to me.