My company shoots sports stuff outdoors. We do Celebrity Charity Pro-Ams for the PGA and private charities.
We shoot action and group shots. It is usually a team of 4 amatures (business guys that paid a ridiculous amount of money to play golf with a celebrity) and the Celeb in the center. We can shoot all day , and with hats and high noon lighting, sometimes blasting them with a flash is the only way to see thier faces.
All that said we use both, we had several 430's, but we've gotten a couple of 580's and now I pretty much keep one on me at all times. The 580 can be too much at times though, so I have to be carefull. But I would rather have too much and find a way to back off or bounce light, than not have enough light at all. They are both great flashes though.
I know my uses for it are almost opposite what your are, but I just typed all they to say you can't go wrong with either.