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I second shoot for a friend and use a D3s and a D800. I use the D3s about 90% of the wedding and the D800 when I shoot from the back of the church with the 70-200.
Everyone has their opinions and likes,etc., but I'd say get a used D3s for not much more than a new D800. The extra low light ability is really nice in dark churches when no flash is allowed. It's at least a stop better than the D800, IMO. And, to me, that pro body just feels much better in my hands.
I used to have a D700 and it's about the same as a D3 for high iso. and when I think about it I'd say the D3/D700 were about the same as a D800 for that. Just my 2 cents.