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I've got the 170-500 (really must get round to selling it) and the 50-500 Sigmas. I got the 170 first and that fits the Pentax. I've not had much chance to experiment with the 50-500 yet, but so far I'd say it's worth paying that bit extra and going for that over the 170-500.
The 50-500 can be locked in position which is useful for ensuring it doesn't drop out to full zoom when you're carrying the camera around. It also has it's own carrying strap to use in addition to the one on the camera.
Those are both big plus points over the 170-500 which does not have a lockable zoom. There's also one major drawback I've found on the 170-500 with regard to the weight - if left to its own devices it will detach itself from the camera, which means that I usually sort of cradle both the camera and lens when I'm carrying them.
I much prefer the weighting on the 50-500mm and it also seems much more responsive and quicker to focus in AF mode.
There's plenty of pics on my site taken with the 170-500, but at the moment, none with the 50-500 just because it's so new to me.
As for weight, honestly Paul, I don't see that as a problem. Sometimes I'll use that lens with a monopod, but mostly I'll hand hold it. Now, I wish I could claim to be the original 7 stone weakling, but I can't, I'm smaller than that, so if I don't find the weight of that lens a problem I don't see why anyone else should struggle with it. I can't comment on the 50-500 in windy conditions, but I have had trouble with the 170-500 on a windy day at Abbey.
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