Which flash for EOS 20D?

Hello everyone, i have been debating whether to buy the Sigma 500 flash (the best one), which runs for about $200, or Canon's 550 or 580EX. Is there really any diference in using Sigma or Canon? Will the Sigma, costing $180 less than the 580ex, produce about the same exposed pics?

This is a pretty debated road you're traveling. I'd love to try the sigma and see how it performs. I've heard it does pretty well assuming we're talking about the same models. Resale on the 550EX is very high but it's possible the 580 might cause a small flood of used 550EX's to hit the market (I just bought a used one today :) )

If money is no limit, I'd buy the 580 and be done with it. If money is a concern, I'd heavily make sure the sigma has the features I want and give it a shot. I'm sorry I can't provide you with real world experience with the sigma. But if you buy it I'd love to hear how it goes.