You should be able to get an original from Sigma relatively cheap.

tick, tick, tick, tick etc. etc. etc.
Yup ticks all the boxes; take no notice of the naysayers. IT IS WORTH THE UPGRADE

And another here

I believe it is down to the density of pixel numbers, because they are physically smaller they cannot absorb as much light. But the difference, according to those that can compare, is utterly negligible and not worth fretting over, what you gain is significant enough to warrant the outlay.

Sigma 150-500 by a country mile. The only other option you would have to reach 500mm (ish) is the DA*300 plus Pentax 1.7x TC for IQ

This is how good it is....
K-5iis & DA*16-50

Well done. Top shot

It certainly is! I just paid £54 for the upgrade only version
I'd say LR5 too it is such a great photo developer and perfect for developing multiple images from single shoot and non destructive too. Photoshop is then the editor if you need it.

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Th e difference is about £50 (2nd hand). The extra money pays for the in lens Optical Stabilisation oh, and it's newer. I had one and it really is great. Only sold it to fund the purchase of a DA*300 that came up at a price I couldn't refuse.
The 150-500 is a great lens and it doesn't matter which one you get because you Pentax has in body OS. The only difference is you can see it working in the lens version, but you still need good hand holding technique whichever shake reduction method you use.