Very helpful and informed comments above. I haven't used the 18-300, but the reviews look promising. Even Lenstip, which is often scathing about superzooms, rates this lens quite well (tested on a Nikon):
Sigma C 18-300 mm f/3.5-6.3 DC MACRO OS HSM review - Introduction - LensTip.com The results in that review are consistent with the user experiences described above.
I'd make two comments (drawing on my own experience with the Tamron 18-250, which was quite a good superzoom):
1. Typically for a superzoom, the 18-300 is about a stop slower than the 55-300 across most of their common range. The Pentax is f4 up to 125mm and then f4.5 up to 200mm, whereas (according to Lenstip's numbers) the Sigma is already at f5.3 by 85mm, f5.6 by 100mm, f6.0 by 150mm and f6.3 by 200mm. At 300mm the difference is f5.8 v f6.3, which doesn't sound like much but makes a real difference in practice. And when you consider that you need to stop down by one or two stops for best resolution, the Sigma does need good light.
2. According to another review, the Sigma exhibits focus breathing. That is, the effective magnification at any particular focal length varies according to the distance of the point focused on. That is common with internal focus lenses. You will see a number of discussions about it here. What it means in practice is that the field of view is wider than you would expect for a given telephoto focal length, unless the lens is focused to infinity (and maybe not even then). The Pentax is not an internal focus lens, and doesn't exhibit focus breathing. In simple terms, the field of view with the Pentax at say 300mm will be the same, whether the lens is focused at the minimum focus distance or infinity. So if you photograph a bird say 5-10m away at 300mm with the Pentax, it will fill significantly more of the frame than if you took the same shot with the Sigma. (On the Tamron, the difference was huge.)
So, for birds and other wildlife, and in mediocre light, the Pentax should perform considerably better. I should add that one upside of internal focus is that the Sigma has a much shorter minimum focus distance; the Pentax is a (sometimes frustrating) 1.4m.