I am in the same exact boat, and have to act soon before the camera arrives on my door step. I am leaning towards a 1D III or 1D IV. I already have a 5D II and want the III mainly for weather sealing and silent shutter, but i think for the money spent it would be better to get the 1D IV.
It's all about what you shoot and what you need your camera to do, and also what lens you have or ones that you are willing to get with your camera.
I shoot outdoors in all kinds of weather, so i am wondering if the weather sealing will be good enough for me. I wonder if i can shoot under an umbrella in crazy rain without much else protecting me, i know the 1D's can and my 40D is better sealed than my 5D II.
I would like to hear if anyone has used the 1D IV with the 100-400mm L, because that is what i will be mainly using. I think using the 100-400 on a full frame could produce some amazing photos though!