If I have to choose between the old Canon 24-105 and the new Sigma variation, I will always pick the former regardless of how good the optics are in the latter. One simple reason: AF reliability.
Addendum: From http://www.dpreview.com/lensreviews/sigma-18-35-1-8/5
"We shot with a range of Canon SLRs, from the entry-level EOS 100D to the top-of-the-line EOS 7D, and all had problems focusing absolutely correctly all the time. This was usually only obvious when shooting at apertures larger than F2.8. In general, we got slightly better results by using the central AF point (with recomposition) compared to using off-centre AF points, but this didn't eliminate focus errors entirely...
We looked at whether the focus problems we saw from the 18-35mm could be mitigated by using autofocus microadjustments. With a Sigma USB Dock to hand, we set about determining and programming in a full set of autofocus microadjust parameters for all of Sigma's specified focal lengths and focus distances (18, 24, 28 and 35mm; infinity, 0.5m, 0.35m and 0.28m). This took several hours to set up, even with specialised focusing targets to hand...
This procedure certainly improved overall focus accuracy when shooting at the distances used for microadjustment. However these are fixed by the software, and there's no option to specifically correct any distance between infinity and 0.5m. Unfortunately though, the vast majority of subjects end up somewhere in between, and we found that the lens still had some problems with focus accuracy even when fully programmed as above."
I have the Sigma 30 f/1.4 DC HSM Art lens. Its AF accuracy leaves much to be desired even though it is optically sound.
This only reinforces my old belief: avoid 3rd party lenses at all costs.