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Hardware wise, the 840K is exactly the same as the 900NC. Software wise, the 900NC has a different firmware on the camera, a different computer driver, which allows a better codec to be used for lower compression when transferring the video stream to your PC. Less compression when transferring the data to your pc is important when planetary imaging.
It is possible to update the firmware on the 840K to make it a 900NC, I have done this to my camera, but I'm unfamiliar with the long exposure mod so I can't say for certain if you can do this on those cameras.
For $10 extra, you are probably better off just getting the newer 900NC camera straight up.
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