I went into my local camera store to play with the 5D Mark II, and they had the 24-70 attached, so I figured, what the heck, I'll play with that lens on it.
I already own the 24-70, so I was nicely surprised by how wide it went but slightly disappointed by how long it was. Sure it was the difference of me stepping a few feet, but that's the first thing I noticed.
I also took pictures of the price tags with the 24-70 that they had in store, and not surprising, it was tack sharp.
The 24-105 however showed some fringing into the text and overall didn't seem sharper.
I hope that helps. These were by no means scientific, but my overall opinion and just what I noticed from the LCD, which on the Mark II, is really nice.