Which is better? One 580 or two 430 
Depends what you want to do with them.
two 430EX as slaves will definitely add a lot more for creativity, 580 on the camera for main flash, one 430 to the right, one 430 to the left. Or most of the light from the side, the 580 on the camera bounced, the other 430 lighting a background. Or whatever, you get the idea.
only if you specifically want as much light as possible directed on a single target should you get another 580, like if you're taking photos of birds at night and you need to get the shot right the first time, and you've got all your flashes mounted on the same tripod as your camera so they all point in the same direction at once. Or if you've got like a bird-feeder set up, and you can arrange flashes whenever you like, i'd personally still go the two 430s.
ps, what's your camera? 7D and (i think) 1D4 can also be a master, you can use your 580 as slave then.
