I have a 580 I and a 580 II. I have never used a 430. A couple of points....
- With the 40D and a 580 II you can control the flash completely through the camera's menu system. This makes adjustments easy, especially when it comes to changing custom settings on the flash gun. These two are a great combination. When (if) you get a second flash - either 430 or 580 then you can set ratios between the flashes all from the back of the camera - very nice.
- If you only get one flash, ever, the "master" controls of the 580 will bring you no benefits, since you will never use them. This means you might be just as well off with the 430 and plenty of change in your pcket.
- The 580 puts out almost a full stop more power than the 430. This is not only useful for distance but also to overpower daylight and give you more options when filling in on a backlit subject.
If you want creativity, flexibility and easier adjustment/control then the 580 is the better choice. You can add further 430s or 580s as you need/wish, to act as slaves. If budget is an important consideration and perhaps you'll not be into flash photography in a big way then I'm sure the 430 is a fine choice.
I assume the 430 offers HSS and strobe effects. If not, you need to factor that into your thoughts. I think strobing will be rarely, if ever used, but HSS can be a very handy feature in bright ambient light.
There's £40 cash back on the 580 II at the moment. I received mine last week
