How does the Sigma 17-70 compare to 17-50?

I really need to go back to a standard zoom and right now can't afford both 30mm and zoom so I got the Sigma 30 up for sale. I'm looking at either the Sigma 17-50 OS or the Sigma 17-70 OS and also possibly the Canon 17-85.

Which would you say would provide better IQ wide open (respectively)? I'm familiar with the Canon lens with their IS and USM focus speeds and I'm happy with that and really enjoyed the 28-135 I had except for not being wide/fast enough. Seems like this 17-70 is a nice compromise, still fairly fast at F4 so beats out the canon in that department.

Any Input?

I loved the IQ. But the variable aperture not so much. Fast AF even in low light

The 17-50 has better IQ wide open, no question. The 17-70 is more than enough for me though. I'm very happy with it, and its more useful to me because of the 20mm extra reach.

Looks like price is the verdict. 17-50 being more obviously.