I have the 24-70/2.8 Mark I. Generally a very good lens used carefully. I never cared for the Canon 24-105, though I used it a number of times, borrowed from CPS. For me, I found it a bit soft, slow focusing and flared easily. I used the new Canon 24-70/4 and found generally a nice lens to use, fast focusing and have had thoughts of replacing my older 24-70 with that lens. For almost any lens, I still adhere to the age old wisdom that you'll find almost every lens is best 1-2 stops from max aperture. Some of my "L" lens break that, but on most it still holds true, so keep that in mind for your shooting.
Finally, the possible deal breaker. Sigma just announced a new 24-105/4 with image stabilization. VERY interesting. They have made tremendous inroads lately with lens quality. Their new 35mm is considered one of the best 35mm prime lenses on the market today. You might want to wait a tad longer and see what this lens brings to the table.