It is the least thing you would care about unless you blow your photos to A2 size.
If you shoot and print on normal size, I bet you wont tell much difference between a good compact and the 5DII. focus on your shooting. Less talking.
the 5DII is a good camera, you should only get the 5DIII if you need one. that is if you need large prints in low light. The 5DII is sufficient in low light. At 6400 ISO using a 50 1.4 your shutter speed in low light is around 1/30 to 1/50 even with +4/3EV as the meter will be confused by the bright lights. So it is shootable. If you find yourself shooting in lower light then you are really shooting things that even your own eyes can't realy see. You are not composing, but using your camera like a photon counting device...taking a picture in the dark of what's out there when you can't see yourself.
I find that my 5DII is just sufficient with the lens. During the day, you are better off getting a better lens. I can bet you that you dont need to spend an extra $3000 to improve your photos. It is the photographer. Moving from 5DII to 5DIII only make some of your pictures better resolution in low light. If a photo is a S___ photo, taking the same S___ composition with the 5DIII will turn that into the same S___ composition boring photo where you can pixel perv a bit more. Not very useful is it?
I'd love to get a 5DIII in the future. But for now, even I have more than enough funds, I won't want to buy anything. I don't need it. It is tempting to replace my 24-70 with the mkII as that is better money spent. But I really don't need that. Most of these improvements are there to make us feel inadequate, and marketing tries to get us consumers justify that we need more, when in fact if we take a closer look, we are most likely to be more sufficient than we thought we were.