580EX, or 430EX, that is the question?
The 580 has a slighly nicer UI, a bit more power, but it's ~$500 after tax, basically. Also, doesn't appear to shoot backwards, only 90 degrees left/right.
The 430 has buttons instead of scrolly wheel, less power (but still good) and faster recharge at full-power, but that might simply be a function of it having lower max output... it definitely does do 'behind', and it's ~$330 after tax.
I'm leaning toward the 430, because I'm not sure I need THAT much range, and I know it does rearward shooting.
However, if the 580 does indeed do rearward, then, it has that extra 'headroom' of more power, and the scrolly wheel interface that I'm very used to.
Both interface some functions with the 5D Mk2, both since E-TTL II as slaves from either flash as the master.