If you delete the trailing "0" from the model number of any Canon Speedlite flash (at least any flash currently on the market) the remaining digits are an indication of the flash "guide number" in meters.
In other words, the 600EX RT or 600EX RT II have a "guide number" of 60 meters. The 430EX II and 430EX III RT have a guide number of 43 meters.
What's that mean?
The "guide number" is the distance at which the flash can adequately illuminate a subject IF the camera is using ISO 100 ... AND the f-stop is set to f/1.0.
The ISO 100 is no problem... but the f-stop of f/1.0 isn't typically possible (Canon used to make a 50mm f/1.0 lens years ago but that lens isn't available today). So why use f/1.0??
Answer: It makes the math easy.
It turns out if you divide the guide number by the f-stop you ACTUALLY plan to use, you'll get the proper distance. e.g. if you use a 430EX II (guide number is 43 meters) but you are shooting at f/5.6, you simply divide 43 by 5.5 and you get about 7.7 (I rounded that value) meters. That's about 25 feet. If the "f/1.0" baseline wasn't used for guide numbers, the math wouldn't work ... so it's really just a convenience to use that baseline.
25 feet may sound like a lot ... and remember this is all at ISO 100 ... you could raise the ISO if you needed to. Each 1 stop of ISO will increase the distance by a factor of 1.4x (technically it's the square root of 2 but we can round it to just 1.4 for simplicity) ... HOWEVER... things get complicated when you start to shape the light by doing things like bouncing it off a ceiling or wall ... or shooting through alight modifier such as a soft-box, etc.
I started in photography (decades ago) as an apprentice for a professional photographer and we did a LOT of weddings. Usinga "side-light" (the assistant holds an off-camera flash) greatly inproves the quality of the photos. But back in those days, the off-camera flash was triggered via a light-slave so that the off-camera flash pops when it sees the on-camera flash pop. Unfortunately it also pops when it sees ANY camera flash pop (meaning all the wedding guests with cameras). It also required line of sight.
The flashes with "RT" in the model name can use radio to communicate ... much more reliable, no false-flashes, and doesn't require line of sight.
I think ultimately you'll want the 600EX II RT ... but maybe you just start out with the 430EX III RT (for financial reasons) and add a second unit later. I switched to the radio system almost as soon as it was released (I had wanted radio for a while and was looking at PocketWizard units ... but they sufferred from some reliability issues). The Canon system gives me full E-TTL II compatability and radio. I now own a couple of 600EX RT flashes and an ST-E3-RT transmitter. You can mix & match all the "RT" flashses... they all know how to talk to each other.
The other nice thing about the 600 series (Canon's flagship flash) is that it has a terminal to plug in an external battery pack. This gives you not only more battery power ... but also much faster recycle time. At a wedding, that's extremely helpful to not miss shots because the internal batteries are getting low and are needing more time to recycle.
Tim Campbell
5D III, 5D IV, 60Da
