Sigma 150-500 vs. Canon 100-400: What's the verdict?

I've been shooting the Canon 100-400 for a few years now, and I am very plesed with my copy. But as any bird photographer knows, you can never have too much reach. I dream about the 500/f4IS L, but I'm wondering how the IQ of the Simga 150-500 compares to the Canon 100-400, esepecially when shooting wide open. Has anyone seen a comparison of these two lenses, in both IQ and AF speed/accuracy?

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• markE

http://www.wingsoflight.com

'Good street/wildlife photography is a controlled accident,
a vision of preparation and surrender materialized.'