I shot with the 1DS2 for years and upgraded to the 5D3 so I have the same exact experience that you have with the bodies. A couple of things...
IF you only have to shoot at ISO 100 then your 1DS2 takes fantastic images. No doubt about it. IF you have to bump your ISO above 400 the images SUCK in comparison. At 1600 they are a complete mess. The 5D3 at 6400 is way better than the 1DS2 at 1600. There are other things that are hugely different as well.
Better screen on the back of the camera by far which in the real work world I need to see if I got the shot I need at a shoot for my clients.
Much better interface, The 1DS2 had the worst interface ever with the strange 2 button controls for everything.
more frames per second
Better dynamic range
The list goes on and on.
The other thing is maybe you need to try a different program to edit your RAW files in. The 1DS2 had a VERY soft out of the camera file because of the high pass filter they put on it. The files sharpened up extremely well in PS but they NEEDED it badly. I would try to do more tests with a different software and also change up your lens. Maybe your lens is calibrated perfectly to your 1DS2 but off a little when mounted on your 5D3. The sensor is not "Sharper" on the older camera. Trust me.
The other thing that is quite noticeable in your test is that the 5D3 shots are exposed brighter in EVERY instance. The test would certainly be more accurate if you match exposure. The settings are irrelevant. Its the final image exposure that matters. Need to compare apples to apples.
At the end of the day if you are are only shooting studio work or landscapes at ISO 100 you would never see the detail difference anyway unless you are pixel peeping. PRINT the images out at 24x36 after making them both look the best they can. Don't limit the sharpening, contrast, etc of one to what you do to the other. That is not what you would do in the real world. You would optimize each image individually for it's intended output. Do that and then lets look and see if the 1DS2 is a better camera. I doubt it. I loved that camera for years and I do love the way the images look from it. That said the sensors are much improved since then and there is more to a camera than a sensor.
Thank you for bringing a little perspective to this old fight. There's a lot more to a camera than its ability to shoot a specific scene at 100 ISO. (And most cameras that have been released over the past 5 or 6 years, including my original 5d can do an awfully good job of that).
Actor Headshots NYC
| Gotham Family Photos
| NY Wedding Photography