I currently own the 430, and i have used the 580, not as much as the 430 since the 430 is mine hehe
The 580 has more power.
580 ---> Guide Number
190 (at ISO 100, feet) / 58 (meters); flash head at 105mm zoom setting
430 ---> Guide Number
141 ft./43m (at ISO 100); flash head at 105mm zoom setting
Now, the 580 has a master feature to control the 430 or other canon slave flash, but, if you only have one unit , it won't do nothing for u, you can purchase the 430 and if you need a second one; after you've made more money, then you can go with 580 and use it as a master and the 430 as slave.
Then again, that canon wireless system is built in but it won't be as good as let's say the pocket wizard or the elincrhom system, since you need to be "looking" at the other flash in order to "fire it"
I got the 430 because it ill looks good on my cam, there are some places where you can only use one flash and i won't like to have some dirty and rusted flash like the vivitar 285 (i own the 285 old one not the new 285HV) wich is great but the looks is not that good.
I say that cuz people see your equipment and will judge you by that (i don't approve that but that's the world we live on)
When you get more money you can buy 2 PW for less than 400$ and get a second handed flash on
ebay for around 60$ or a NEW brand of flash and use them as slaves.
Hope this helps!!!