I have owned both and ended up with the 200-500. One thing you'll want to check from the very beginning is making sure you have as sharp copy wide open at maximum focal length. Both lenses have some copy variation and could leave a bad taste in your mouth if you end up with a bad copy. That being said, I ended up with the 200-500 for a few reasons: faster aperture, significantly less vignetting wide open, much lighter (I can hand hold it significantly longer than the 150-600 S), better image stabilization, and better bokeh. Wide open sharpness is excellent with both lenses, and I think the decision should ultimately come down to whether or not you think you need that extra 100mm, or if you hand hold your lenses for a significant amount of time. I, for one, rarely use a tripod or monopod which made the 200-500 choice a no-brainer for me. I've gotten sharp images hand held down to 1/30 sec. at 500mm with the 200-500, something I would never be able to do with the Sigma given its weight and seemingly lesser image stabilization.
Here are a couple of sample images from the 200-500 from this past weekend:
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