the 970 sli performs about 10-15% faster.
the advantages of switching to a 980ti are more ram, less noise, less heat, no sli bugs/conflicts and much less power consumption
That being said - i strongly suggest you wait until dx12 is out as it has new features that may affect multi-gpu configs in a big way. That way you can see what changes the new api brings before plonking down another 50k+.