I have both cameras. In my opinion the 5D2 beats the 7D in terms of IQ and DOF control. In all other respects the 7D is the superior camera. While on paper the 5D2 does have the IQ edge, in practice is it something you would really notice for your normal photographic needs? i.e. Unless you print/display at large sizes, could you spot the difference in the final product (rather than when pixel peeping)?
That said, because of the IQ the 5D2 is the camera I reach for first for most of my photography. There are times when I will choose the 1D3 or 7D in preference such as....
- When I know I will be focal length limited and the 7D's higher pixel density will be more useful to me than a load of wasted sensor real estate;
- When I know that the 5D2's AF, frame rate, ergonomics or build will not be up to the task.
For shooting people and landscapes it's pretty much a slam dunk for the 5D2. For sports, wildlife and action the balance tips towards the smaller sensored, faster, better featured bodies. They all have their place. I used all three at the BWC on Sunday.
Another, simpler way to look at it - I usually use the camera with the largest sensor I can fill with my subject, unless the AF or operational speed of the camera is not up to the job. Often I will take two cameras with me. That gives me more options. I can use the 5D2 for slower, wider angle shooting, or for shallow DOF portraits and also be geared up to shoot fast action with a longer lens. It may weigh a bit more, but it saves faffing about with lens changes and missing the shot, dropping a piece of equipment or getting dust in everything.