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I'm using both at the moment, although not for weddings. There really isn't a whole lot of difference between the two unless you're pushing out to ISO 3200. The focus on the 1D2 crushes the 5D. While not normally key for weddings it can be an asset if you have a habit of trying to grab unstaged photos while doing the set shots -- the focus acquisition on the 1D2 destroys the 5D. Either way, you're using two different battery systems. Shoot RAW and with a good converter things should still normalize out. While not wedding work, there are still quite a few sports shooters using the 1D2 to shoot under lights.