I have both. IMHO there's no reason to spend the extra $$$ to get the 600EX unless you plan to get the flash off camera and use the radio transmitter function. My 580EX II is usually used as my on-camera flash, and I have my 600EX RT set aside to just use with the ST-E3 RT.
I don't know what the ST-E3 RT functionality is. Can you explain a little bit? Do you have any reference where I can look up to understand better the difference?