I was in a similar situation a while back and replaced a long-in-the-tooth D200 with a D3100. I do miss the build, but the high ISO performance, higher resolution and build in video (I'm starting to experiment) were the clinchers.
The big advantage to having two identical cameras is the control set. In your case, high ISO is a key feature, but honestly the real solution to that problem is faster lenses. Short of breaking the bank on a full-frame, I'd rather sink it slowly into f1.8 lenses, of which there are a goodly selection in FX and DX (i.e., 35mm f1.8 DX).