Why 24-105 sigma?
Canon 24-105L is as good and proved lens....?
The Sigma is optically quite a bit sharper than the 24–105L, and exhibits much less distortion—particularly at the wide end, where the 24–105L's distortion can be objectionable at times. Based on the numbers I'm looking at, Sigma's 24–105 outclasses Canon's so completely that unless weather sealing is a must-have, Canon's infamous distortion monster (the 24–105L) really isn't a serious contender anymore, IMO.
In fact, the Sigma 24–105 actually approaches the performance of the 24–70L series in many respects and (if I'm reading the numbers correctly) beats them
in center sharpness. Admittedly, it can't match the corner sharpness or low distortion of the 24–70L lenses, but it really puts into stark contrast just how bad the 24–105L is, at least when compared with other L lenses, and when compared with what is possible in that focal range.
Sigma really caught Canon with their pants down on this one. IMO, Canon needs to either update their decade-old lens design ASAP or risk forever ceding the focal range to companies that actually take it seriously.