If i were in your shoes, my question to myself world be whether the 17-55 will be put to good use or just sit in the bag or storage. If it will see minimal use, I'd rather let it go and get another lens.
The 24-105 is a nice lens. With the 6D, the f4 won't hold you back much unless in very low light situations and you got more range as well. Very nice walkaround lens.
I got myself a 70-200 f4 IS and planning on a 17-40 (i see that you want to get a 16-35). That UWA is verrrrrrry wide on FF. One would need to take a different approach to composition.
Edit: You may also consider fast primes like 50 1.4 or 35 1.4 or 85 1.8. The Sigma 35 1.4 looks like a very nice glass. But for the same price you can get the 50 1.4 and 85 1.8. All depends on what you want to shoot mostly.
I was thinking of the 16-35 2.8 because the range isn't much different than the 17-40 but is faster. I'll be walking around Japan in a few weeks and thought I'd need something wide. The 17-40 would be easier to obtain price wise but I thought the 16-35mm 2.8 would help the T1i in low light situations since I'm giving up the 17-55. I'm basically replacing the 17-55 which only works on the T1i with the 16-35 which works on both the 6D and the T1i. Not sure what to do now. Maybe get the 17-40 and 85mm 1.8?