The 600EX-RT has many improvements over the 580EXII. From the back panel display, the expanded zoom range, the hotshoe contacts and the flash head itself to the huge plus of ETTL radio control.
If you're trying to save money as you get started then buy a 430EXII. It's a better flash than the 580EXII, quieter, faster, a heck of a lot lighter, only a half stop less power and you can continue to use it when you realize you want the 600EX-RT.