Image Stabilisers: Optical or Mechanical?

Which are better, optical or mechanical image stabilisers? For years this has been a tough question because there have never really been two cameras that are exactly the same but with different stabilisers. Now, thanks to Sigma, we can compare and contrast.

If you want to measure how effective an image stabiliser is, there's really only one way to do it: turn it on, take a series of photos at progressively slower shutter speeds until the shots come out blurry, and then compare the results from each series to those obtained using no stabiliser. So, between the Pentax K-5's mechanical stabiliser and the Sigma 50-500mm's optical stabiliser, which produces the sharpest photos, and at what shutter speed?

> Camera Image Stabilisers: Optical or Mechanical