Its really hard to say. The ISO performance I would say is equal...which is crazy considering how old the 5D is...the 5D is very impressive in that part. For IQ...about the same, but I might give a slight edge to the 1D3. As far as AF............the 1D3 rocks the socks off of the 5D!!! Ain't no doubt about it!!!! It is amazing how fast the AF is...I was actually out back just shooting an Osprey and it locked on and it was truly amazing what I got!
I didn't need a lot as far as action this wedding but I can tell you, in a dimly lit reception hall, the 5D would struggle at times unless there was a large contrast...the 1D3 doesn't seem to care! I also had only a handful of out of focus shots...the 5D, I had more than enough.