Boy, lots of thoughts on this one! I use a 24-105 F4 as my walk around lens on a FF body. With the ISO capabilities these days, unless I am shooting Aurora the difference for me of F2.8 and F4 is a moot point.
The F2.8 24-70 and 70-200 seem like a common setup. I sold my F2.8 70-200 recently and kept the F4 version. The 2.8 lenses are just total beasts to carry. It all depends on what else your gonna carry, and what you're willing to carry. Sometimes I have a 500mm lens with me so carrying more lenses that is F2.8 and heavy is just not appealing to me.
It's not an easy choice, but I agree take you time choosing the glass. You will most likely upgrade your body several times before the lenses. I actually dropped my 24-105 the year before last (grr). I like it enough that I spent like $300 bucks or more for Canon to fix it.
Here is a shot in Lightroom at 100%.
This Swallow was taken at ISO 1600 on the Mark II which is not known for it's high noise capabilities
. I did do a quick pass using noise reduction software though. It's a tad pixelated from the JPEG compression but the full size image is silky smooth.
