Thus, the main difference is that with the 7D you have 18 MP, and with the 5DII you have 8 MP.
Help me out here, neuro. My 5DmkII has way more than 8MP...it's 21.1. What do you mean by 8MP?
The 7D is a 'crop sensor' - that means it samples only a small portion of the FF image circle. That's the basis of the 'reach' argument for APS-C (red is FF, blue is APS-C):
If you crop the 5DII image down to the field of view of the 7D (e.g. crop the red image to the blue image, above), you'd be left with 8 MP of your original 21 MP.