You mean the
Tamron 17-50mm f2.8 and the Pentax
DA 16-45mm f4?
Pentax is wider and darker. Quite a difference between f2.8 and f4. Also, I think the Pentax is a bit odd, since it extends at the wide angle. The Pentax has also been discontinued, so you probably can't find any new copies. Maybe you can look in the sample photo gallery and see some photos. The rendering of the two lenses is pretty different, but hard to say that one is "better"
Lots of people like the Tamron because it has great image quality, but they say its build is not quite as great. The Pentax has character, but its a beloved oddball. I don't have both of those lenses to compare them, though. Doubt either would disappoint you, and both should be significantly better than the Pentax 18-55mm
Edit: But back when I only had the DA L kit lenses on my DSLR, I went for a prime lens next. DA 35mm in my case. That is a more significant IQ boost.