Depending on the price of the 6D + 24-105 kit, it might make sense to buy the kit now and sell the 24-105 later for a profit. Right now, the difference between the kit and the body only is 550. Used 24-105s go for a bit more than that, so you should be able to turn a profit when you want to trade in the 24-105 for a 24-70 VC.
Or you can get the kit now, sell the 24-105 and your Sigma 17-50 and get the 24-70 VC. It's more economical to get ultrawide zooms for crop cameras, and a Tokina 11-16 or a Sigma 8-16 or a Canon 10-22 will be less expensive than a 16-35 II.