I have a Nissin Di866ii which has a similar GN to the 600EX of around 60. Compared to a 430EX, the increased power is noticeable and handy. But the extra power isn't enough to give me dramatically improved capabilities. Therefore, I'd make my decision on the cost vs functionality of the flashes. Are there any features that are must haves? The 600EX is a well regarded flash. Aside from the power output, there are a number of important features in the 600EX that interest people - the main one being the radio triggering (assuming you're looking at the 600EX RT). The radio triggering will form the basis of Canon's flash units for the next XX years, and is something to consider if looking to future proof your system.
If you want my opinion (bearing in mind that I'm just a weekend hack who has never taken a professional photo in my life), if you just want a flash that works well, the 430EX is a great choice. Good mix of power and features. They are well made and should just keep going. It can also be used as slave if you get into more advanced flash set ups. Therefore, even if you move on to bigger and better things, this will always be a handy flash to own. (BTW, there have been a lot of rumors about the 440EX-RT which should be even better.....but, of course, waiting around for fictional products to materialize is rarely a prudent strategy.)
The big advantange of the 600EX RT is the radio triggering, but you only get the full benefit in a multi 600EX flash environment. Which is costly.