hi bunbun,
I was given the opportunity to use both L-lenses you mentioned, namely the 24-105 F4L, and the 24-70 F2.8L.
I have also done extensive reading up on the 17-55 F2.8 as I believe in doing a fair amount of homework before finally putting the dough down for an expensive lens, so please do allow me to share with you my opinions regarding this.
I currently own the 24-105 F4L, and I made this decision based on several factors.
1.
Reach. This lens has far better reach than all three lenses. I am more of a street photography person and this reach is good for me. I do sneaky shots of people sometimes haha.
2.
Aperture. The down part of the 24-105 would certainly would be the F4 part. One stop would make quite a difference, but unless you're shooting in low light conditions ALL the time, the difference would not be LIFE CHANGING. Outdoors, F4 is MORE than enough really.
3.
Bokeh. I choose to seperate this section from the aperture part because if you don't already know, allow me to tell you more about how bokeh comes about. There are 3 factors to bokeh, they are namely:
a) Focal Length
b) Aperture
c) Distance from object
Based on these 3 criteria, to attain the most bokeh, you would want to use a lens at its widest aperture, longest focal length, and get really really close to your object. In that respect, the 24-70 and 17-55 certainly have the aperture bit, and the 24-105 has the focal length bit. So really, the aperture is not EVERYTHING when bokeh is concerned.
4.
Image Stabiliser (IS). Last but not least, I would like to talk about the IS system on the 17-55 and the 24-105, is to DIE FOR. Old school photographers will go on and on about how they dislike IS and all that, but really, I say harness technology

. The IS system gives you
3-STOPS worth of handholdability. The difference between the 24-70 and 24-105,
1-STOP difference. Handholdability between the two? You do the math.
But I would like to remind you that IS does NOT freeze motion. In other words, in low light conditions, you're more likely to get sharp blurry shots. Sharp because you're able to handhold at insanely low shutter speeds (my personal longest was 0.6 seconds) but blurry because non-stationary objects (i.e. people) will most likely MOVE in that time.
Conclusion
If you haven't already guessed it, I'm in full support of the 24-105, because I was in your situation half a year ago.

I laid out the cards, and decided what I really needed. Before everything, I would like to point out that the 17-55 F2.8, is constructed using
L GRADE ultra dispersion lenses. Just know that it's what makes the L lenses legendary. Which is also why it's so expensive and incredibly SHARP for a non-L designated lens.
That being said, it all boils down to your needs, to your comfort and most importantly, your style. Don't worry about image sharpness. You're not going to be printing poster sized images, so that wouldn't matter. When you're doing like 4R prints, seriously, you can't tell the difference between these lenses. They are all good.
Just my many many cents worth. It was long, but I hope I helped in your decision.