I would get a 430EX II and an ST-E2, or two 430EX II's .... which you can just about get for the price of one 580EX
Some of the benefits quoted by Al Brum above I don't think are correct, you can certainly do manual zoom on the 430EX II from what I recall and I don't think the recycle times for a given flash output are different either.
http://www.digitalrev.com/article/canon-flash-comparison-speedlite-580/NzcxNA_A_A
ST-E2 is one of Canons most under appreciated pieces of kit and would allow you to get both flashes off camera and still give you AF assist on the camera and allow you to fire both your flashes remotely.