I was in a similar position until just recently.
I've been only using my onboard flash, and even then...not too often, so I kept thinking "I hardly use flash, so I'm hardly likely to need the extra features of the 580 over the 430".
I was undecided at first whether to go for the Jessops 99.99 one, or the Sigma 500...In the end I decided to go Canon for the guaranteed compatibility, and to buy a flash that was literally dedicated to my camera.
I think if money wasn't a concern, then I would have gone for the 580, purely to cover the chance that I might need its features in the future.
But saying that, I got my 430 from
Onestop Digital for 142.99, which was about a 85 saving over the 580. I could have used my voucher to get 35 cashback on the 580, leaving it at a price difference of about £50.
But then I thought "Am I *really* likely to need the extra features....can I justify the extra cost?"
In the end I decided, no, I couldn't, and plumped for the 430.
I think my answer to your question would be....if you can afford to go for the extra without batting an eye, then go for the 580. I've read reports of people missing the extra features and upgrading later.
BUT, if you're not likely to ever use the extra features or need them, or if the price difference is too great, then you really can't go wrong with the 430.