Thanks everyone. I wanted to make sure I wasn't missing something and this confirms that.
I would consider the following:
- Is build quality an issue to you? The 430 feels decidedly flimsy compared to the 580. Also, is weather sealing an issue to you?
- Is strobe flash a requirement for you?
- Do you need your flashes to be able to put out exactly the same amount of light? - If so, stick to 580EXIIs
- The point about how much light output you need has already been mentioned
- The 7D can't trigger a flash that is behind the camera
- Will you work a lot in bright daylight - i.e. overpowering the sun - if so get every bit of power you can
- Do you use light modifiers on your speedlites? They will eat up a lot of flash output.
- Do you need a PC sync socket on all your flashes - if so, the 580EXII is the only choice for you.
FWIW, the speedlites I have access to are a 580EXII, a 430EX and a 430EZ (yes!). I make do with those out of necessity at the moment, and most of the off-camera flash work I do is manual (using RF triggers and optical slaves).
What will probably be the trigger to buy another 580EXII is for my wife's work to be able to do decent off-camera TTL flash work, where the power of the 580EXII is a factor - especially using a light modifier (probably a brolly) and balancing flash with ambient light.