I had the Sigma 24-70 before my Canon 24-105 and wasn't impressed with the Sigma at all, very soft until stopped down to f5.6 and never really sharp. Selling it was a good move.
I've shot a number of weddings with the 24-105 using it throughout its range and often wide open at f4 and had no issues with the results at all. In fact a week ago I had a pair of 24-105s having just bought a 5D with one and they both produce fine results.

I do sometimes wonder whether to swap to a Canon 24-70 f2.8 but I think I would miss the is for low light work and certainly would miss the 70-105 range for my studio work