However 1DMkII will have faster FPS, way better focusing, weather sealed, much more rugged and the vertical grip is included. Would you trade those features for high ISO noise performance?
BTW the FF goodness of the 5D has some special mojo / magic that crop cameras do not have.
I haven't used a 1DII, but do use a 5d for weddings. I shoot tons of photos at 1600 and never have a problem. You can even shoot at 3200 if you get the exposure and focus correct.