I don't have the Pentax 60-250 but I have Sigma zooms (although not the 18-250). Check out the lens reviews on the database and you will get a rough idea of how lenses compare. Most users think that the Pentax 60-250 is very good. The Sigma 18-250 is good but probably not great in the same way that most users consider the 60-250 to be. Any zoom lens will have some compromises, and those compromises tend to become greater as the zoom range increases. The zoom range of the 60-250 is about 4x; for the 18-250 it is about 14x. So one would expect the Sigma 18-250 to be "not quite as good" as the Pentax 60-250. The drop off with super zoom lenses is generally at the long end. But that is not to say that they are not good enough for every day photography. Many (myself included) find that they do the job very well.
The Pentax costs about 3 times what the Sigma does, so we should expect it to be better. But is it 3 times better? Only the user can say that. If you don't need to make big prints of shots taken at long telephoto lengths, it may not matter to you.
On the other hand, the Pentax 60-250 is a constant f4 which is well ahead of the Sigma 18-250 f3.5-6.3 in light gathering power. A full 1.5 stops more light at the long end might be the decider for you if, for example, you spend a lot of time photographing wild life in low light. So it really depends on what you need - or what you will use.
If the slower speed is not a problem, the (relatively) new
HD Pentax DA 55-300mm f4.5-6.3 ED PLM WR RE may be worth a look. It is well rated and also costs about a third of what the 60-250 does. It may be about the best of the consumer lenses with that zoom range.
Others will have opinions to add. Weigh it all up and - most importantly - make your choice and post some pics.