I cannot understand the actual benefit of having an effective increase in the focal length of a lens when using a crop sensor.
It simply provides a little extra "reach" when shooting with a long lens. On my 40D, a 300mm lens has the equivalent field of view of a 480mm lens, but with the same maximum aperture. So my 300/4 becomes, effectively, a 500/4. This can be a useful feature for some kinds of photography -- wildlife, landscape, photojournalism, etc.
Could you shoot with a full frame camera, and simply crop out the middle, and get the same result? Sure.
The downside to this field of view issue is, of course, shooting wide angle. On the 40D, my 16-35 becomes, at best, about a 28mm equivalent. Not very wide at all. I would have to purchase a 10-22mm lens to get the equivalent field of view on my 40D.
So we're left with the idea that different cameras have different strengths and weaknesses, and one chooses the appropriate camera for the job. This isn't a new concept -- photographers have always had different cameras for different assignments.
Etienne, none of us can tell you what camera to buy. All we can do is provide some information about our experiences with these cameras, and what we might do differently now. All I can say is that the crop sensor cameras do provide excellent image quality in and of themselves. Does the 5D Mark II provide better and larger files than the 40D? Sure -- but then a medium format digital back provides even better image quality than that, and you can go crazy spending more and more money to get smaller and smaller returns. Given your print size requirements, I think any current D-SLR from any manufacturer would provide excellent prints.
Good luck.