550EX
If you cant afford it, or wouldnt mind the lack of canon and a tad lower build quality then get the sigma.
I have my 550EX and it's just...awsome, truely.
Here are some examples:
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(yes...all the light here was provided by the flash)
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this one not possible on anything less hten the 550 (or sigma)
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It looks cool too

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Again, only the features of the 550 (and sigma) can do this
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