To me, the faster F2.8 (throughout the entire focal range) of the Tamron was worth more to me than the IS. The 17-85 starts at F4, but goes to F5.6 as you zoom out to 85. My personal preference is to get the fastest lens you can for the situation, instead of relying on IS. And IS is only good if the subject is still. My kids are never "still", so the faster lens is needed. To go faster than F2.8, you'll need to go with a prime instead of a zoom.
Look at the Bob Atkins review of the 17-85, and look at the Aperture chart. By 50mm, the 17-85 is at F5.6 minimum. Although I give up the 51-85mm range on the Tamron, I prefer the constant F2.8 from 17-50mm on it.
Not saying you should do what I did, as I may not entirely understand your shooting style or preferences. I just wanted to expand on why FOR ME the Tamron is a better choice. Given that I think buying the 17-85 was a mistake FOR ME, I just don't want to see anyone else do the same. For me, it was a lack of knowledge at the time. My focus at the time was completely on focal length range of the lens. I wanted the extra 1mm wider than 18, and thought the extra reach from 56-85 would be nice. It is nice for the extra reach outdoors in daylight. But to me the 17-85 is a "good lighting only" lens.