I'm not really familiar with converting guide-number to my shooting. But I am familiar with STOPS, so how many stops is the 580EXII greater than the 430EXII?
If GN = distance × f-number, then all things being equal (105 zoom setting) at 10 meters the 430 should produce f-stop of 4.3 and the 580 an aperture of 5.8.
Considering the nearest f-stop equivalents of f4 and f5.6 respectively, you should be getting approximately 1 stop difference or double the power.
Where the extra power of the 580ex is helpful is when the light is bounced, diffusers are used, or the flashes are used at extreme distances or outdoors for fill in high speed FP mode.
I know that between a 430EX(ver.2) and a 580EX (not ver.2) measured with my pro level Minolta meter was a "Real World" difference of .4 or 4/10 tenths of a stop. So not quite a half stop of light loss. It may be greater with the 580EX2.
There are power measurements, and then there is the real world. Yesterday's wedding required a lot of full power fill flash(possibly the worst lighting situation I've every been in). My 580 was acceptable, but my 430 would not have been. So if this is for fun, get the 430 and enjoy it. If it is for work, get the 580(hell, get a used 580mk1) so you have it when you need it.
