When I first saw this topic, I expected the usual "full frame is professional/APS-C is amateur" rants. Thankfully, it hasn't turned out that way (although there have been some, "my sensor is bigger than your sensor" postings)
Still, I'm not sure I'm any closer to coming to any clear conclusions. Granted, it can boil down to personal preferences and needs, but I still have a difficult time parsing the thread for obvious distinctions between APS-C and Full Frame.
It sounds to me that the differences are rather subtle and seem to have as much to do with the "look" of the image and shooting style (narrower depth of field, for example) than with any clear, visible difference in image quality.
Many of those posting have indicated that they feel full frame offers superior performance for landscape and other static or semi-static subjects. But, I wonder if that translates into practical or theoretical differences. If one shoots with the idea of enlarging the images to 16x20 or larger, are the differences visible? What about 11x14 or 8x10. What about those who shoot primarily for publication? Are the differences visible once an image is translated into print?
I shoot with a 7D and to be honest, I have a hard time imagining that the practical differences would be that great with a larger sensor. But, then I don't really go for the oversize prints and my ultimate objective is to get work published in books or magazines.
The main problems or complaints I've had about the 7D in terms of image quality has been that I do feel that certain areas of an image under certain conditions can have visible noise on-screen. (Although this does not always translate into being visible in print). For example, if I have an image with a large amount of plain, cloudless blue sky, upon close examination, some of the images will appear mottled in the sky (instead of completely smooth.) But, some of this results from the wide differences in brightness between a sky and other portions of the image (exposing for the one means under or overexposing for the other and attempting to repair in processing).
My other frustration has been chromatic aberration. Back lit subjects with sharp edges that have significant fringing that can't always be fixed (minimize the magenta and you maximize the cyan, for example). Again, I don't know if that would be improved by a different sensor or not.
Bottom line of this rambling post: Do those who use both a 7D and a 5d II really see a difference in the final product? And if so, do you feel the differences are visible at say 8x10. Do you feel the differences are visible at ISO 100-400 or only at higher ISOs?
Now, I know that preferences are personal and can only be ultimately decided by the photographer after having used both cameras, but I do wonder if the 5DII/7D dual body owners would be willing to give an honest assessment of the differences they can actually see in image quality.