Before buying the 24-70mm 2.8L, try to rent (hire) one for a bit, and actually walk around with it, all day. This is a quite heavy lens! Some of these heavy pro zoom lenses will not balance well on lighter cameras, and too much weight toward the front can make make the whole ensemble seem heavier than its actual weight. (I consider the 400D to be a very light-weight camera; I used the USA equivalent XTi regularly for a while, and it is still available for me to use.) My idea of a walking-around lens is something a bit lighter than the 24-70mm 2.8L, though I would love to own one for shooting at some social functions.
A heavy lens may not be such a bad thing, depending upon how one carries the camera while walking about, and how one holds the camera while shooting. Does one primarily let the camera hang by a neck strap, or does one hand-carry the weight of the camera? If the weight is mostly borne by one hand, is it the right or left hand? The left hand cradles the lens, whereas
the right hand usually grips the camera body. All of this affects what one
finds comfortable to carry and shoot. If one bears the weight of the camera
primarily in the right hand, a heavy lens on a light camera is more likely to
I have not settled upon one perfect walk-around lens. Much depends upon where I plan to walk on a given day, the light level, and how open the area. My wife is a much more accomplished photographer than I, and she believes in the 18-200mm zoom, accepting the distortion at the extreme ends of the zoom range, and the aperture limitations.
As I ponder the two lenses mentioned in the original post, I would lean toward the 24-105mm as a walk-around lens choice, and the 24-70mm 2.8 as more of a working lens to use for a planned shoot. The nearest lens to these two, that I own, is the 28-135mm EF that was packaged with my first 7D as a kit lens. I would want an ultra-wide-angle lens, such as my 10-22mm EF-S, to be available, with any of these choices, as neither 24mm nor 28mm is very wide on a crop-frame camera body. Actually, I can very much enjoy my 10-22mm, alone, for a day of walking about!