I"ve never used the canon, but I have been using the Metz 58 for awhile now, shooting weddings, people etc. and it works great, a bit cheaper then the canon model and seems to be just as good from what I can see. note the metz 58 has the small frontal flash as well which you can set to a ratio of the main flash, helps in situations to lighten up under eyes etc. if you have the main as a fill and the secondary flash a quarter the power of that.
If i remember correctly the metz is a bit lighter then the 580exII? but don't quote me, just trying to remember holding a friends. who i believe also liked the battery compartment in the metz better then his 580exII?
Canon 7D gripped - Canon 10-22mm 3.5/4.5- Canon 24-70mm 2.8 L - Metz’s Mecablitz 58 AF-1 - Canon 580EX II - Presslite Vertex - Qbox 24