From a wedding photography perspective, having never had the second card slot option on my previous 5D's and 5DII's, I see more value in affording two bodies going into your career than I do in spending the same money on the 5DIII alone. Now that I've gotten used to the 5DIII, I definitely prefer it for the way it handles and the color it produces, but I literally had to sell two 5DII's to get one. That being said, I've always shot with a body on each shoulder and I've never missed a shot due to card errors anyway. In a sudden 'oh-crap' moment I could always swap a card from the other body much faster than swapping lenses. Paying attention to how many shots are on your card leading up to important moments is part of the job anyway.
Someone mentioned shooting the whole day on a pair of 128GB cards, which I think is smart in terms of redundancy,
but there can also be drawbacks to not spreading the day out over several cards, if for instance your body is stolen at the end of the night ( It happened to one of my colleagues at a high-end venue...1DmkIII) then all of your pictures are gone. Odds are low, so I guess it all comes down to what we perceive to be the bigger risks.