I am asking myself the same question and haven't reconciled all the variables to advantage/dis-advantages yet. I'm really interested in what everyone has to say too. I wanted to post in this thread for two reasons...
1) I don't have time during the week to "read around" in the forum, and only check in on the threads I've posted in; so I need a post here to catch/read any posts.
2) I wanted to ask TanGU why you're not including the Tamron 70-200mm f/2.8 in your options?
I see a K-5 in my future. So I'm (now, more) comfortable with a slower f/4 of the 60-250. And it would be my first, if not only DA* / Weather Sealed lens. If the Sigma or Tamron had weather sealing, it would certainly work in their favor; it works in Pentax's favor that only they offer WR.
edit: p.s. I was sure when I read reviews of the new HSM Sigma lens on this board that a few said it worked with a TC. (?) Hoping those weren't the older non-HSM lenses posted in the HSM review thread.