I have shot extensively with both. The 1dsmkII is far more responsive, but I hated the LCD - when I was shifting studio lights around it was very hard for me to judge the results on the screen. The interface is also for the birds. (yes, I know, pro series, but the mkIII and mkIV abandoned the crazy two finger approach, right?)
At low ISO, the images from both are similar to me. Some have said that skin tones are better on the 1DsmkII, I haven't seen that. At high ISO, at the pixel level, the 1dsmkII is CERTAINLY noisier. But if you scale it down to 12mp, it's essentially the same. The 1dsmkII clearly has a bit more resolution.
The AF system in the 1dsmkII is wonderful, but I found the center point to be LESS sensitive than the center point of the 5D. I think that's well known. Again, in good light, it's terrific.
To me, it's not worth the extra cash unless you need the much improved AF Servo focusing or really rely on the outer AF points.
I traded the 1dsmkII up/down? for a 1dmkIII - not as nice of a sensor, but my customers can't see the difference and the cam is fast and accurate and the high ISO is astounding.