I'm not overly familiar with the Metz flash, but I know they make decent equipment. But just a couple of thoughts to consider:
I wouldn't pay much attention to the guide numbers unless you've read an independent analysis of it. The numbers companies publish aren't all standardized. I'd just assume they both have _about_ the same max power level.
Can the Metz be used as a master flash with the Canon TTL system? If so, then that's a minor point in favor of the Metz. Personally, I'll never use optical trigger systems again, but it doesn't hurt to have the capability.
Personally, I'd be wary of the touch screen on the Metz. I'd just prefer physical buttons, for ease of use, durability, and battery life. But maybe they're a nice feature; I don't know. That said, I don't love the buttons on the 430exII either. I find it difficult to get my finger onto the little buttons. I much, much prefer the scroll wheel of the 580ex series.
I assume that you want to have an eTTL capable flash? If not I'd recommend completely different flashes (Yonguo). But for eTTL, my first choice would be to find a good deal on a used 580exII. Fantastic flash, lots of capabilities, and stronger. If you can't find that, or want new, then I'd base my decision off the above. I'd probably go Canon, but that's only because I don't know anything about the Metz flash. If I did some research and people had good things to say about the Metz then I'd go for it. I'm not married to Canon in any way. Other than my lens mount 🙂
