Personally, were I you, I'd stick with the Canon 580EX-II ... it offers more systems choices, and virtually every modifier is readily available for it. The off-camera cord for the Canon maintains the red focus assist at the camera, as does the STE-2 ... which is an advantage IMO. The Canon's W/A diffuser covers out to 14mm where the Metz is 18mm.
According to specifications the Metz recycles in up to 5 seconds, where the Canon is up to 6 seconds.
I do not have a Metz 58 AF-2, but do use the Leica version of the Metz 58 AF-1 ... The AF-2 is basically the same flash as the AF-1 with some tweaks (like a new metal fast mount hot shoe design). The claimed recycle for the AF-1 and AF-2 is the same ... 5 seconds. I find that to be an ambitious claim, it seems slower than that when fired full power.
The other thing I do not like about the AF-1 is that the head rotation isn't full 180º left and right ... it is really irritating when shooting a wedding. I'd check if that has been fixed on the AF-2 ... AND ... the head itself is too loose to hold any modifier other than a light one ... even a Lumiquest makes it flop forward all the time ... it needs more secure click points.
In favor of the Metz, it does have a secondary flash to illuminate the eyes when bouncing off a wall behind you
When I shot Canon, and needed fast recycle, I used a little Quantum Turbo C, and it was very, very fast.
Hope this helps,
-Marc
P.S., IF I were shooting Canon and was using an aux. battery for fast recycle, I wouldn't use either Canon or Metz ... I'd use a Quantum TRIO E-TTL bare-bulb unit which produces a far superior quality of light ... and recycles in 1 second! (or 1.5 seconds with the little Turbo C battery). The range of modifiers is mind boggling. Just a thought.