Sorry to dissent here, but I have the same choice and am leaning towards the Tamron. The Tamron is available for $429, quite a bit less, and is 10mm longer and a full stop faster. I am sentimental enough that I could live with giving up 2 of those 3 for the Canon, but not all 3 (especially a full stop of aperture). I just can't see paying a lot more for a less capable lens.
Note that I have a 350D and no plans to get a full frame camera - FF is obviously a huge benefit to some, but not to me. In fact, the way I look at it is that Canon is hamstrung in this particular competition by the need to cover FF. I bet the Tamron would be more expensive and F4 if they had to cover FF.
D