hold on a second this is a bit unfair - the thread is about comparing sensor size - a much fairer comparison would be the 40d and the 5d
like for like - both current models.
i currently own a 1ds (soon to be sold here) and a 1d - the full frame has pros and cons on the plus side the 1ds has higher resolution, but it also shows all of the faults with lenses, the smaller sensor is more forgiving.
But the 20D has the same sized sensor as the 40D? Why not compare a 40D to a 1Ds MKIII? The build quality of the body and menu system have no impact on picture quality after all.
As you say, at the end of the day it's like comparing apples to oranges and yet people persist in doing it. Both have their 'pros' and 'cons' :shrug:
I tried a 5D and decided to sell my 40D to purchase a 5D because I feel that for my needs the 5D was a better choice. I get a shallower DOF, better high ISO performance and better colours and detail on A3 prints.
Other people that maybe have a keen interest in sports or wildlife for example would almost certainly choose a 40D/D300 if the likes of a 1DMkIII or D3 are more than they'd like to spend on camera.
One other thing for those that believe that a full frame camera is more demanding on lenses, this is not necessarily true and brings us back around to some of the pros and cons of each format.
Lenses being used on a crop camera need to be able resolve at a greater number of lines/mm than when used on a full frame meaning which means a cheaper lens mounted on a 5D may well give better results (eg.Sigma 24-70 f/2.8 EX)
The flipside is that as the full frame sensor makes use of all of the rear element on the full frame compatible lens it will show up any softness in the corners though this tends to show much more when shooting at wider apertures when it's arguable that edge/corner sharpness is of little interest to the shot.
So it's arguable that the ~£600 saved by buying a 40D over a 5D could theoretically be swallowed up in having to buy more expensive lenses to obtain the required lines/mm resolution
