The short answer is that the gain of staying with the D3:
1. Less Noise
2. More bit depth.
The negative is slightly less resolution.
The D3 aside, I find the Canon 5D provides better IQ then the 1DSM2 (for it's less noise), with exception of resolution, which is slightly more with the 1DSM2.
I rank them in this order:
1. D3
2. 5D
3. 1DSM2.
In other words, stay with the D3...because going to the 1DSM2 is going backwards.