if it counts for anything i previously owned the sigma 17-70mm you're talking about. it's got a mighty nice range, but i found that it had a LOT of trouble focusing... especially with a moving subject (flowers in the wind was what i had most trouble with...) and it has a LOT of trouble focusing on things with little contrast... and it was also slow to focus.
if you're shooting a moving subject, you have to move with it, or else it might move out of the focus area before the lens finishes focusing, and starts focusing on something that was in the subject's place. this was super frustrating when i tried to get flower shots.
not gonna lie, i think my nikon kit lens performed better than it when it came to focusing. the lens wasn't extremely sharp either. maybe around the same level as my kit lens? i dont know if it's just 'cause i missed the focus... but it seemed a little less sharp than the kit lens at times.
maybe i just got a faulty lens, but i returned it for the tamron 28-75/2.8... i love it. it's been on my camera 90% of the time after i got it. it has a decent but awkward range, but i barely shoot at anything under 24mm anyway. the constant aperture was a plus for me.
the 28-75 is a pretty damn sharp lens too... when stopped down to around f4-5.6... it doesn't focus as nicely as the kit lens (most 3rd party lenses don't focus as well as original manufacturer lenses though), but it has a lot less troubles than the sigma. it's lighter than the sigma, smaller than the sigma... but doesnt feel as durable as the sigma.
i've heard great things about the 17-50/2.8 from tamron, but i've never tried it... the 28-75 works fine for me :] but that's only 'cause i don't really shoot at focal lengths under 24mm... the range might be awkward for a lot of people.
anyway... hope this helped, happy shooting :]