DA70 or 17-70mm or...

So I think I'm ready to move on from my kit lens to something else.

I'm using a K10D and generally take landscape / nature photos--I like to take walks and shoot. More and more lately, though, I have been taking pictures of people at gatherings (birthday parties, company gatherings, etc), so I'd like a lens that can also handle portraits fairly well. After researching a little and reading through some threads here, I think either the DA70 or Sigma/Pentax 17-70mm would be a good next step for me. I was wondering, though, which of the two would be a better choice, or if maybe I've missed something else entirely that would be also good--I'm very open to suggestions! My price range is ~$500 or so. I'm hoping this lens'll last for some time (I probably won't be able to buy another lens for a while), so I wanted to get advice and feel good about whatever I end up getting.

Thanks,
Brandon