Taking the 5D3 plunge

you won't it ask crisp to the edge as with the 17-55 so in that sense the 24-105 won't compare, that said 22MP vs 10MP would still give you a lot more overall detail captured over much of the frame even if the 24-105 is just a trace softer even in the center, so overall the pics with it for landscapes might be better in the grand scheme of things but at 100% view it likely won't appear to take as much advantage of waht the 5D3 sensor can do as the 17-55 does of what the 40D sensor can do

AF will be wayyyyyy better oh soo soo much better

much better user interface too

and video

and yeah better high ISO for sure, especially when not distance limited at least

the one pseudo negative is that the 40D has some of the least low ISO shadow banding of the canon cameras so you can really push the images as far as the random read noise allows, while the 5D3 has a lot of vertical banding so you can't push them to the random noise limit, whether the usable DR of the 5D3 is actually worse than the 40D I don't know, but it's probably not any better even though it should be