Hi Heinrich. I was in similar dilemma before trying to choose between the two. I wished there were some help available at all.
DPR only has 2 to 3 threads regarding Bigma example shots and the rest of posts come from C*non and N*kon websites.
APO 170-500 5-6.3 is quoted not as great as 50-500 4-6.3 DG EX HSM: Not as sharp and colour not as great.
I bought mine recently and I tend to use it between 400-500. This lens has been designed to cheat the camera body so that even f6.3 can do AF on k10d. Somehow the lens gave the signal of f5.6 to the camera body to autofocus while in fact the aperture was 6.3
There is a wide speculation regarding Bigma soft at long end over 430 mm. This was not the case with me as long as there is enough light! The major limitation is the aperture of 6.3 at long ends. HSM is fast but to me, not fast enough. The colour is great in the same par with sigma 70-200 f2.8
Imagewise, not as 3D and punchy as pentax lens colour renditon.
Eventhough it is called Bigma, it is not that big and only 1.4 kg weight. Not heavy to be honest. Quite handy as a lens since it covers a big range of zoom.
I will give you a few examples here. I had not shot the images often enough yet.
1/1000s f/8.0 at 500.0mm iso400 full exif
1/500s f/6.7 at 420.0mm iso400 full exif
1/1000s f/8.0 at 500.0mm iso400 full exif
1/250s f/8.0 at 500.0mm iso400 full exif
1/750s f/8.0 at 500.0mm iso400 full exif
I hope the images help.
James