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Both are very solid lenses. The Canon 17-40L has amazing colors but isn't as sharp as the Tamron 17-50, and the 17-40L is also only f/4 compared to the Tamron's f/2.8. Just a couple things to consider if you end up buying one of these.
I absolutely love my Tamron 17-50 but I kind of wish I would have saved a little more for the Canon. The tamron is incredibly sharp but the contrast on the Canon is much better and the overall build quality is much better. Plus you can't go wrong with L glass. The Tamron is definitely a great lens though, I looked around a while on ebay and found mine for 360 new from a US seller.