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I'd stick with canon since you already have the glass. :) I recently purchased a 40D for 940$ from B&H because of this exact reason. Hooray for Canon instant rebates!

It all has to do with quantity of tones in terms of stops. Here's an excellent article on 14bit cameras, and the benefits over 2 to the 14th power vs 2 to the 12th power of bits.
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