I'm considering buying a flash for my Canon DSLR. I noticed the 600EX recently came out. What are the pros and cons of the new 600 flash versus the other two mentioned in the title?
Price vs Radio wireless, I have a 600EX-RT and I love it. The wireless radio is the big upgrade. It should support far more complex wireless flash operation, though currently it only works with more of the 600EX-RT flash units. (Presumably there will be more devices in the RT line down the road.)
I don't know if the flash bulb moves to adjust the guide number in the 430EX or not, but I believe it does in the 580EX. I know it does in the 600EX-RT which is a pretty nice feature. The menu system on the 600EX-RT is also very nice and will work very easily through the camera's menu system as well on newer cameras.
So main pros for the 600EX-RT that I notice are the newer design (backlit LCD screen with easy to use controls), wireless radio sync (no line of sight limitation/longer range/more control) and cons are higher price.
If you plan on taking advantage of the radio wireless TTL capabilities of the 600EX-RT, either now or in the future, you will either need another 600EX-RT or the ST-E3-RT to use as a transmitter. Some third party wireless triggers, such as those offered by Pocket Wizard, will also allow wireless TTL operation of Canon flashes including all three that you mentioned in your question. Each option adds considerable expense to the price of a single 600EX-RT.
If you plan on using the flash mounted on camera or connected by a hot shoe cable the 580EX-II or the 430EX-II offer more bang for the buck. The biggest difference between the 580EX-II and 430EX-II is the maximum flash power for each unit. The respective guide numbers for the 600EX-RT, 580EX-II, and 430EX-II in feet @ ISO 100 are 197, 190, and 141.
Built-in RT radio triggering -- The 580EX/430EX and Mk IIs can only do optical slaving, which is limited in range and line-of-sight when used outdoors in bright light. Radio triggering is much more reliable, but until the RT system, typically required add-on triggers. Built-in radio triggering simplifies the amount of gear and batteries you need to pack/manage when doing off-camera flash work with speedlights.
Additional features -- Canon updated their wireless slave system with the introduction of the RT system. They expanded the number of groups from 3 to 5 (A-E, not just A-C). They increased the number of RF channels from 4 to 15, and then added four-digit access codes on top of that, giving 15,000 discrete digital channels). They added "Groups" mode, which allows individual groups to be individually turned on/off and to be operated in mixed Manual, eTTL-II, or MULTI modes. The system, however, like the older optical system, still cannot communicate 2nd curtain sync or zoom settings to remote flashes.
Better UI/UX -- The newer flashes have dot-matrix LCDs, allowing for soft button labels, so the confused multiple-function labelling of the limited number of buttons on the older units is done away with. Custom functions don't require you to use the camera menu or carry a little cheatsheet to set them. And there is a new dedicated button for putting the flash into radio slave/master modes, and the ability to change the color of the backlight on the LCD screen depending on whether the flash is in slave or master mode. Overall, the controls are easier to operate and less cryptic than the older models. (See: a comparison video between the 580EXII and 600EX-RT made by Syl Arena for Canon Europe).
Cost -- Like all newer systems, this gear is more expensive than the older units.
Requires a newer camera body to take advantage of all the features -- the RT system was introduced in 2012. So, only camera bodies that came out in 2012 or after can "speak" the full RT system. Pre-2012 bodies, such as, say a Canon 5DMkII, cannot take advantage of the new RT features that the optical slaving system doesn't provide (e.g., Groups mode, ID codes, the newer channels, or groups D & E).
Annoyances for Switchers
- No OEM RT support for studio strobes or older Canon EX flashes -- Unlike using, say, PocketWizards, or other 3rd-brand of radio triggers, Canon has no additional units to slave anything other than one of their new RT speedlight units in the system. To add a studio flash or older non-RT EX speedlight into your combination of lighting gear, you either have to go with 3rd-party add-on triggers, or stack 3rd party triggers with an RT-transmitter. Yongnuo, Phottix, and other 3rd party trigger manufacturers are beginning to offer RT-compatible slave units that can be used with studio strobes, but if you got a 600EX-RT, you're probably someone who prefers the reputation, build-quality, and service warranty of OEM gear.