It depends how you use the flash unit. I have Metz at the moment and I am going to upgrade to 580EX. Metz (at least the one I have) does not support E-TTL, only TTL which requires manual control every second shot. On the other hand, do not overestimate E-TTL II, it is only uses distance information when aimed directly to the subject, therefore, not distance is used when bounced. How often do you photograph people aiming stright to them?

The white balance information is not that usefull as well, as it will transfer it only when the flash unit detects that the battery going low and therefore the strobe temperature changes. If you use flash with complex lighting or/and as fill flash, it is not going to help. The result will be close to accurate only in flash is your main light.

I would still recommend 580EX over any Metz unit due to the fact that it is really integrated with the camera and it is bi-directional. Not all the E-TTL features supported by Metz due to proprietary Canon E-TTL, they reverse-engeneer and not always 100% correct and not all the features.

In general, if you do not have 550EX, go for 580EX as you will get a bit smaller unit, a bit easier to use controls on the back and a bit faster recycle and a bit more powerfull output. So, there are quite a few "bits" that make 580EX better flash vs 550EX, but there are no "wow" that will force 550EX users to run to a shop to upgrade.