First of all I'd like to say that I'm a hobbyist, not a professional photographer, and only recently did I get into flash photography. My current setup is 550D + 24-105L, but I'm getting one of the 5Ds (ii or iii) when the latter is available. I'm mostly shooting general "walkaround" photography with some lean towards press photography (e.g. political meetings and etc.). I also shoot at weddings, family reunions, that kind of thing. So now I need a flash since many of these events tend to happen indoors, and I'm currently deciding between 430 and 580.
From the looks of it I'll have only one flash for the time being (I'm not a studio photographer, more of an action one), and 580 seems to be easier to operate. On the other hand it's much heavier than the 430 and costs more. Do you experience this additional weight problem, and do you feel that the price difference is justified in this case?
I can relate to that. I've been shooting quite a few political events lately ('tis the season...). I have a 580EXII, 430EXII and an old Metz 45. Camera is a gripped 5DII.
The nicest outcomes I get with the old Metz (obviously all manual). And it's actually quite comfortable with the old-style handle - at least for horizontal shots. The fastest and easiest, however, is the 580EXII. So that's been the weapon of choice lately.
The 430EXII is nice in that it is a bit smaller and lighter and under certain circumstances a bit more consistent (there have been plenty of controversial discussions around that so I'm not going into the details). BUT: the 580EXII has the extra power reserve should you need it and the buttons and the little wheel are just way better.
So, if this is truly a weight question I wouldn't bother and go for the 580. If this is a budget concern: the 430 is a very good flash. You could also consider the Metz 58 AF-2 which is usually cheaper as the 580 and offers pretty much the same features (has nothing to do with my Metz 45 mentioned above).
Those are pretty much the alternatives I guess. Or wait and see what a new 590EX (or so) may have to offer it's really released within the next few days. May also be a good time then to snag up a used 580EXII. Also: there's nothing wrong with the old 580EX predecessor I guess. Some folks actually prefer those.