Good quick analysis of the two here
Personally, the PC connector on the 580 is the deal breaker.
Flash
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Canon Speedlite 430EX II vs. Canon Speedlite 580EX II
Wednesday, January 7th, 2009Now that’s a fast flash!
Friday, June 20th, 200880 attoseconds (billionths of a billionth of a second) long. Wow. Link
Now if they can only get that into a speedlight (And a shutter that can handle it). :0)