You RIG mate?,
SLI :
Pros:
Good performance,
Costs you lesser as you have a 960 already,
Cons:
PSU constraint,
Energy Constraint
Assuming your PSU wouldn't be able to handle the SLI[Just sayin
]
GTX 970:
Pros:
Lower Power consumption when compared with 2x gtx960
Cons:
Costs you a bit more[Depending on the availablity]