About this debate between Canon and Tamron, I think at the end of the day, it's what one wants. 10 people saying that the Tamron is "better" than Canon while only 1 says otherwise doesn't mean that Tamron is indeed better. I mean what if all the 10 are amateurs and the tests were crudely done? Moreover, how were the tests performed? Are they objective in the approach? Even if so, I think it is fair to say all these tests are simple that -- tests. What gives the final kick is the practical use of the lens, not just in the laboratory... and that of course includes the use of skills involved in photography.
You won't go wrong with Canon lenses, that's for sure. Where the Tamron might score a tad higher in terms of sharpness (I'm not supporting any brand here, neither have I done any tests to verify this), Canon kicks its butt by providing USM and IS function, which may be all that matters in deciding which to get if you do a lot of handheld shots in fairly low light.
Personally, if asked to choose, I'd favour more with the Canon for its IS. The Tamron and Canon have wide zoom range and they are slow lenses at 135mm, f/5.6. The IS could prove useful. But that's only me. If there are other intentions, the Tamron might prove to be the one for you.
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