On paper, these lenses look almost the same, but the Tamron sadly isn't as big a bargain as I would have hoped.
If you're going to shoot a lot of moving subjects, don't get the Tamron. The AF of that lens isn't really great for that – I had more keepers with even the 55-250IS in those situations.
But, all in all, the Tamron is not a bad lens for the price. Try both (like I did), and you certainly know why the Canon 70-300L is more expensive. The Canon really plays in a different league when it comes to image quality and autofocus (and of course, it's also weather resistant and its zoom ring turns in the right direction
If you have ever tried a 70-200L f/4 IS: The 70-300L performs very similar.
: I also use a 7D.