I was ready to buy the 430EX but decided on the 580EX I (used) and added the 580EX II subsequently. I'm glad I did go this route. The differences for me that made me decide on the 580 are the guide number/power of the 580EX's and using them with umbrellas, no white bounce card in the 430EX, AF assist for the 430EX is 9 pt and 45 pt for the 580EX and recycle time is much faster with the 580EX. I would consider a used 580EX I rather than the 430EX II. The weight of the 580EX I is similar to the weight of the 430EX II.
You can also use an external battery pack on the 580, which reduces cycle times, and increases battery life. You can get generic versions from your favorite Chinese dropship site (I've had good luck with dealextreme) for less than the cost of the batteries you put in them.