Mendel Leisk , Jun 16, 2012; 11:06 a.m.
The-Digital-Picture is a good source for lens reviews and comparisons. This page allows you to compare target shots of both lens (assume you mean Canon's non-L version):
The Tamron looks pretty good, at least at the outset 70mm and f4.0. If he's done the target shots for the Tamron lens, he's maybe done a review as well. Have a look around that site.