Of the two, while having no experience of either, I would go for the Sigma as it has the OS image stabilisation which is very useful if you want to hand hold. I had the old Sigma 170-500 which is probably not as good as the 150-500 but on a reasonably bright day gave pretty good results as long as it was on a sturdy tripod. For the money I don't think you could do better than the Sigma 150-500.
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