The 5970 would offer better power savings due to the lack of redundant (and therefore, wasteful) power circuitry otherwise entailed from dual GPU solutions (SLI/xFire).
The 5970 would also hold it's value better then 2 5870 cards.
I'm trying to think of an arguement for a SLI/xFire setup, but I am a dualGPU/single card proponent myself (9800GX2 FTW!).
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