What are you going to be doing with the flash? Will it strictly be on-camera, automatic flash? Or would on-camera, full manual flash be good enough? Or maybe you want to get it off the camera, and then again, auto or manual?
If you're just going to be using on-camera, TTL flash, get the 430ex. As Big Mike said, the 580 is nice because it can act as a master to an off-camera flash (430 or another 580), but if you decide you want to go this route you can always add a 580 to your kit later.
Personally, I use old Nikon flashes. I mostly use them off-camera, but occasionally I'll put the Nikon SB-28 on my Canon. I usually only do this when I'm doing indoor shots and just need some bounce flash. The only downside here is you're working in full manual, so it's a little bit slower (but not much).