I dropped my 580 in the parking lot and sent it to Canon for repairs. I'm hoping nothing is wrong. I just broke the little red piece in front of the sensor. It is still under warranty. They didn't ask how it happened and ok'd the repair. Send it. It takes 10 business days normally.
About the Sigma or 430ex. I have the 430 and it is quite capable, but not as powerful as the Sigma. If you know you need that extra power, I'd say go with the Sigma. But the 430 with the sync ability is really great. I use the 550, 430 and 550 all the time as strobes. Good Luck!