I had to make a similar choice a couple of months ago.
Went to a local photoshop and tried them out both on a 60D.
The 24-70 just feels good, I found statement of it being heavy to be exagerated.
The mm between 55 and 70 are very nice to have for portraiture, the way I shoot makes 55 mm a little to short, having both a 100/2.0 and 24-105/4.0 compensated in that respect.
I did not have anything better than 5.6 in the 17-24 range wich was a limitation that did hurt.
Both are optically excelent and I did not expect that I would be hold back by either of them.
Handholdability under limited light pulled it for me towards the 17-55, stationary subjects can be captured with 1/30 and still be sharp because of the IS.
Finally went for a 17-55 and did not regret, my copy lives up to its reputation and I am very happy with it.
It might even be that because of it, and no equivalent on FF that my next cam will be a 7D and not a 5D.