The Tamron 18-250mm is the brother of the Pentax DA18-250mm. Simply the DA18-250mm is a Tamron lens. Very little difference between these two lenses. If you already have the DA18-250mm, I would not consider to replace it by the Tamron 18-250mm.
The Sigma 18-250mm OS HSM is an newer more expensive lens. There have been some good user reports (see for example some threads in this Forum). But the lens Optical Stabilisation (OS) is redundant with the K-7 camera Image Stabilisation (IS). The lens OS and camera IS cannot work together: you must switch off the OS or the IS (*).
(*) On the pros and cons of OS vs IS, see the excellent article by slrgear.com:
Image Stabilization Test: Olympus E-520 SLR Body - SLRgear.com!. Using an Olympus E520,
www.slrgear.com compared the camera IS and the lens OS. The findings are clear: "
The bottom line on the Olympus E-520's IS system is that it turned in a superb performance, very much on par with the best lens-based IS systems we've looked at".
Personally I have the DA18-250mm with a K-7. I considered the Sigma 18-250mm as an alternative to the DA18-250mm, but decided against because of the more expensive price, but also the HSM. I use my camera outdoor in rugged conditions and there have been still some concerns about the HSM robustness of some lenses.
In your place, I would keep the DA18-250mm. It is a great lens that you could complement with a series of prime lenses. For example, a fast prime for low-light conditions is definitely a great addition. (I bought the Nokton 58mm f1.4 and I am very pleased with it.)
Hope that the comment will help.