Originally posted by hcc The lens OS is redundant with the camera SR (shake reduction).
There is an interesting article on in-camera image stabilisation vs lens OS:
Image Stabilization Test: Olympus E-520 SLR Body - SLRgear.com!
The outcome: "
The bottom line on the [camera]'s IS system is that it turned in a superb performance, very much on par with the best lens-based IS systems we've looked "
In your case, your camera has already IS (the SR system); you do not need an OS-lens and you would better off by getting the non-OS lens if the optics is identical to the OS-lens.
The linked test/comparison is irrelevant to the subject, unless you can find a good side-to-side test of Pentax SR vs. Olympus in-body stabilization. (I'm not sure if this is possible due to the differing crop factors, which may in fact affect the relative benefit of in-body stabilization vs. lens-based stabilization.)
The general rule of thumb I've seen is:
In-body stabilization tends to do better for shorter focal lengths. Lens-based IS tends to do better for longer focal lengths. Even the article you linked comments on this aspect - "was about as good as that of one of the best lens-based IS systems we've tested, namely that of the Canon 70-200mm F/4L IS. And at shorter focal lengths, it did better."
The OP is looking at lenses with significantly longer focal lengths than tested. (400-500mm actual focal length - longer than even the EFL of the lenses tested.)
Also keep in mind that in-lens stabilization is not just about stabilizing your shot - lens-based IS stabilizes your viewfinder image, making it easier to frame the shot AND making the autofocus system's job much easier. I have a Sigma 18-250 with OS - the OS really helps with focusing.
On the other hand - I don't know about the 150-500, but for the original 50-500 - the non-OS Bigma was EX grade glass, while the BigmOS is NOT EX-grade glass and numerous reviews say that there is a clear image quality difference (not just a naming/designation difference.)