I have a 24-105L which was my go to lens on my crop camera. I switched to FF, kept the 24-105L and it really shines on a FF. 24mm is surprisingly wide on FF. I know I should not have been surprised by this but this really is the prefect walk around lens for a FF camera. It's on about 90% of the time. You pick up a tone of depth of field switching to FF and F4 is plenty fast for a 6D. I have a 5DIII that can practically see in the dark and I hear the 6 is better. It would be hard to buy a 24-70L 2.8 I since the mark II is supposed to be so much better. BTW, if you are concerned with the weight the single best thing you can do is loose the stock strap and spend $50 bucks on a Black Rapid. Takes all the weight off your neck and allows you to walk around for hours. makes you look like a pro too!