I'd say avoid purchasing either if IQ is an issue for you. From what I see simply browsing around on the web, neither produce absolutely stunning results. They work, but not much better than your average cheapo kit lens. If you're looking for a walkaround lens, try the 18-2XX Range lenses, they reportedly have better IQ overall, and will usually run around the same. If it's the telephoto you're looking for, try investing in the Pentax 55-300 and perhaps swapping between those and a kit lens.(If you have one)
Though I'd say if you're intent on getting one of them, to get the Tamron, just based on the reviews on the site here, seems to be better overall.
Best of luck with whatever you choose to do!
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