I have had experience with both lenses and if I was to choose between these 2, I'd definitely go for the 17-70, as it's just a better lens. I had a Tamron version of the 18-250 and the main reason I got rid of it was that at the wide end, it tended to vigenette (sp?) more than I liked. The Sigma doesn't do that. Another thing about the Sigma that I liked is that it has a good closeup (1:2.3) feature at 70mm. If you find yourself needing anything beyond 70, then you'll be well-served by lenses like the DA50-200 or DA55-300, among others.
HTH,
Heather