The best flash to get kind of depends on what you plan on doing with flash now and in the future. The 430 is an all around good flash and will suit most people's needs. It's plenty powerful enough and does most of what the 580 will do. If you want to use two or more flashes and have them wirelessly triggered by the Canon system then i don't think the 430 has the ability to be a master (trigger), only a slave. The 430 is less powerful than the 580, but still plenty of power for most needs. So you could get the 430 now and if you want wireless Canon ETTL with another flahs you could get the 580 later.
If you were to only shoot the flashes in manual mode it would be much cheaper to get some third-party brand flash and trigger them with some radio triggers like Pocket Wizards.
E-TTL is Evaluative Through The Lens. In a nutshell what this means is the flash will figure out automatically how much power to deliver to expose your shot properly. Of course as with all 'automatic' settings this system can also be fooled.