[markgozz]

harvmcb I can't comment on the Sigma 17-70 because I don't have one but as a rule of thumb a lens with a shorter zoom range will give better image quality compared to a lens with a longer zoom range ( as Cathus said )
The Sigma 18-250 I can comment on as it was my first lens when I got my camera , it is a very handy all in one lens which can deliver very good images in the right conditions . It's sweet spot at 18mm is about f/16 so if your shooting hand held you need a very bright day or you need to mount it on a tripod to get the best out of it , if you open up the aperture to speed things up on duller days you get noticeably softer images even at the 18mm end so when you start to zoom out things start to get softer still ( I read somewhere that the sweet spot at 250mm was out at f/40 , not sure how true this is because I've never tried it ) . The question you should ask yourself is will you be happy to loose image quality but only have one lens to carry about or would you prefer to get the best out of your camera with more dedicated lenses , I know that if I was to start over again I would buy the sharpest lenses that I could afford even if that meant waiting to save up for each lens .

If you look at my gallery all my shots are taken on either the Sigma 105mm macro or the 18-250 O/S

Mark