I have both. The 17-70 f/2.8-4 I picked 5 years ago, shortly after I bought my 60D and it and lived on that camera until I sold the camera. It lived on my 50D until recently when I picked up the Canon 15-85. The 24-105 came with my 5D3, and that lives on my 1DV. I can't see ever giving up my 24-105. That being said, as the others have pointed out, if you're going wide for some shots, the 17-70 will suit you well. The 24-105 is a great lens IMO but a bit too tight for landscapes, etc on crops. Image wise, I can't tell the difference between them. But I'm in it for me, not for others, so I'm maybe not as conscious of IQ, CA and soft corners, although I always felt the IQ was good, CA was minimal, and corrected in post, and I don't recall soft corners on any of my lenses. Hope I didn't add to the confusion.
I have read and heard that the newer copies of the 17-70 were even better, but I have no experience with them, so I can't offer an opinion.