First, consider whether you are buying a second body, or a backup body. The former is one that you would use essentially interchangeably with the D3. The second is one that you would get out only if the D3 fails and you need to finish what you're doing.
If you're really getting a second body, the D3 is really the way to go, because by definition its control system is IDENTICAL. You don't have to think about which body you're using, you just react. I shoot two bodies at motorsport and equestrian events - you really don't want to goof up where your fingers are going. I imagine that weddings are like that too. I've tried the gamut from a D2h and a D100 through a D2h/D2x combo (identical except for one control that isn't used much) and now a D3/D2x. The D100 is just way too different, and I'm not talking about resolution or sensor - I mean button placement, responsiveness (37ms vs 75+ms response etc). The big D2 and D3 bodies are all close enough to essentially ignore differences once they're set up, and I can just reach down and pick up the other one and shoot. I couldn't do that with a D700 either. In fact, this is one major reason why I don't have a D800 now.
If on the other hand you're really getting a backup, it doesn't matter much. The D700 backs up for a D3 very well, and while you might not be smoothly getting the right buttons immediately, you'll work it out shortly - particularly OK for an amateur. (Although weddings don't sound amateur to me: they sound like you have to get the shot, which to me is the definition of a pro.) Given the usual pricing of D3 and D700, I'd ordinarily go for a D700 as a backup. But with the delta being only $250 or so, it seems a nobrainer to go for the second D3.
Worth noting that there are two functional differences that may be material: the D3 shutter is rated to 300k cycles, the D700 to (I think) only half that. To me that alone is worth the $250. The other difference is that the D3 has two AF processors, while the D700 has only one. It doesn't make much of a difference most of the time, but if you use the complex AF modes (51-point, group dynamics, etc) things might be different.
I guess we should also remember that the D700 has a built-in flash capable of being a CLS commander, too. That might be worth considering in some situations.
_____ Brian... a bicoastal Nikonian and Team Member
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